Ah! My Goddess: When Urd splits into good and evil parts, she actually does this to herself, saying "if you're the real Urd, you should know where Keiichi has his dirty magazines, etc." Followed by answering those questions.
Cool Old Guy Joseph Joestar is forced to rely on this in JoJo's Bizarre Adventure Part 3. After being killed by Dio Brando and having his blood drained by the vampire, he undergoes an emergency blood transfusion to try to bring him back to life. Upon awakening and seeing Dio's corpse, his first thought is to play a joke on his grandson, Jotaro, and pretend to be possessed by the vengeful spirit of Dio. Jotaro is ready to punch his face in, but Joseph narrowly avoids being killed a second time by answering questions from Jotaro, such as "Who starred in the 1981 remake of Tarzan?" (Bo Derek) and "Who sang "Eat It?" ("Weird Al" Yankovic).]] As Jotaro puts it. "Only you would know such stupid things".
Used as a red herring in Detective Conan, where Jodie is routinely seen using a catch phrase of Vermouth ("A secret makes a woman a woman"), the Black Organization's master of disguise (who bears some resemblance to Jodie). Turns out that Vermouth said the same phrase to Jodie after killing Jodie's parents.
Played with much earlier in the series, in episode 2: Conan reveals he knows about the mole on Agasa's butt to try to convince Agasa he's really Shinichi in a kid's body—but Agasa just assumes Shinichi's been blabbing. Then Conan uses a Sherlock Scan to demonstrate that he really is Shinichi.
In an early episode of YuYu Hakusho, Yusuke's deceased spirit possesses Kuwabara's body to tell Keiko that he's coming back to life soon, so he needs her to take care of his body. She refuses to listen to him, so he suggestively compliments her "Nice skirt!" (a callback to a scene in the first episode where he flipped her skirt) and grabs her breasts. This causes her to slap him across the face while shouting "Yusuke, you jerk!" out of habit. Since that wouldn't fly outside of Adult Swim, the Bowdlerised Toonami version cuts out the boob-grabbing.
In Baccano!, Rachel and her associates at the Daily Days figure out that the Rail Tracer was actually the train conductor, because the Rail Tracer knew Rachel was a stowaway and asked for her ticket.
Later, Claire Stanfield reveals himself to Rachel as the aforementioned rail tracer by making a comment about her ticket purchases.
You know how that demon that started the whole thing is always saying "maa, ii"/"well, no matter"? You know how that guy Ronnie who works for the Martillos is always saying the same thing?
In Sailor Moon, Chibi-Usa knows that a monster disguised as her mother is a fake, because her mother calls her "Small Lady," not "Rabbit," which is the Black Moon clan's name for her. A similar situation occurs in the Sailor Moon: Another Story game, where Nergal disguised as Sailor Pluto is given away because she calls Chibi-Usa "Princess," while Sailor Pluto would normally use the same "Small Lady" nickname.
Also in one of the final episodes of Sailor Moon S, Mistress Nine reverts to using Hotaru's regular body to get SM to give her the Holy Grail (the "Purity Chalice" in the U.S. TV broadcast version). When SM finds her among the ruins of the Mugen Academy, she realises the person she is talking to is not Hotaru when the latter calls her by her civilian name, Usagi (the real Hotaru never knew her identity).
Pretty Guardian Sailor Moon has a great, if narmy, scene, where Sailor Moon exhausts herself trying to protect Tuxedo Kamen and nearly falls down the stairs. Tuxedo Kamen catches her and asks if she's an idiot. SM automatically retorts with "who are you calling an idiot" before realizing their dialogue mirrors the one between her and Mamoru after the latter saved her from being run over by a car. Cue dramatic unmasking.note One-sided, as Mamoru already knew her identity by that point.
In Fullmetal Alchemist, Edward and Ling are wary of one another since either one could be the impostor Envy. Ling proves himself to Edward by offering to recite, in perfect order, all of the items on the room service menu from the hotel where they were staying. He then starts to press Ed's Berserk Button by making a reference to his shortness; when Ed pitches his characteristic tantrum, Ling knows he's the real thing.
Zigzagged when Bido recognized the second Greed due to his signature speech. While he correctly identified Greed as a result, Greed himself didn't recognize Bido until after killing him and suffering when his memories came back.
Later on after Greed's Anti-Heroic BSOD Ed discovers Ling in the hut they used as a fallback after Mustang's guys, the Elrics, and Ling's party worked together to capture Gluttony (interestingly the last place in the normal world Ling was seen alive, really, since the two of them got to Father's base by way of Gluttony's stomach) and Ling's falling over and asking for food is taken by Ed as a sign that it's really him.
Played with fantastically towards the grand finale, when Hawkeye expertly uses this to her advantage against Envy when they get split off during Mustang's Roaring Rampage of Revenge.
Hawkeye: When it's just the two of us, the Colonel calls me by my first name.
Envy: [changes back to his normal form] So the two of you have that kind of relation-
Hawkeye: I Lied. [shoots him] But it was very nice of you to fall for it, Envy.
Akumetsu's mask in the eponymous series really isn't that good for concealing his identity, what with leaving much of his face including his distinctive hair and eyebrows and most of his grin visible, but it's by his 'ya know' that our initial viewpoint character recognizes him in the first arc. When he comes in and coincidentally kicks off his terrorist career by chopping up the Dirty Old Man who'd just hired her in her new career as a teen prostitute with an axe. They're both normal high school students again the next day. (Her for real, him only apparently.)
Of course, he doesn't care all that much if he's recognized, since Akumetsu has a policy of dying on the job and any one of him probably has an alibi, and even if he doesn't who's going to believe that the terrorists are a clone army, even after dissecting dozens of identical bodies? Also, the one who saved her wasn't the one she actually knew, even though they're all bizarrely similar for several dozen identical twins brought up separately, right up to the 'ya know.'
The scene gets a callback later, though, with one of the not-important-and-enough-to-be-targetted minion with the underwear on his head tracking the girl down because she said 'Shou!' when Akumetsu came in and said his Catch Phrase.
In the final episode of Durarara!!, Mikado guesses that someone in a chatroom is Kida. The response? Grading Izaya on a score of 1/10 with the square root of three, a callback to an earlier scene with the pair.
Happens with Tetsuya in Metal Fight Beyblade. He wears a (actually very convincing) disguise to try and steal all of Ginga's beypoints, but is recognized at the last minute because of his alias 'Nigel Crabbypants'. Being openly obsessed with crabs does have a downside! Who knew?
In Soul Eater, when Maka first shows up in the black room, Soul wonders if she's just an image created by the Little Demon. Her response is to hit him over the head with a book, something only she would do. After that, he admits that "Yeah, it's Maka."
During an early Naruto arc, Sasuke gives Naruto and Sakura a lengthy passphrase to identify them with, since there are enemies about who might impersonate them. Naruto goes off to relieve himself, and upon return identifies himself with the correct phrase. Sasuke promptly beats up the imposter, as the real Naruto is too dumb to remember a passphrase that long. Predictably, when the real Naruto shows up, he admits that he forgot the passphrase completely.
Defied during the fourth Ninja World War arc. Chouji proposes something to this effect to make sure that they aren't being impersonated by white Zetsus, but Shikamaru says that the clones have been able to fool the ninjas with this method, as "Sometimes a lucky guess ends up being right." Only Naruto has a foolproof way of telling the real onesfrom the fakes.
In Zone of the Enders: Dolores, i, James realizes that the Orbital Frame Dolores is connected with his supposedly dead wife when he hears her singing a lullaby she used to sing to their children.
This is how Leafa finds out Kirito is really Kazuto in Sword Art Online when he blurts out of his plan to save Asuna from Sugou's clutches as this reminds Leafa that he has feelings for Asuna.
In Gintama, this is how the Yorozuya gang ends up identifying all of the players they run into while playing MonkeyHunter as people they know outside of the game. Some examples include Katsura using his Catchphrase, and Hasegawa lamenting the fact that he's unemployed.
Katsura is especially prone to this in general, as referring to him by anything other than his name frequently prompts him to use his Mad Libs Catchphrase in the format of "It's not *noun*, it's Katsura!". He's managed to blow his cover on multiple occasions because of this.
Daredevil: in the first appearance of DD's ex-girlfriend Elektra, he reveals his identity when he addresses her by his old nickname for her.
In the original W.I.T.C.H. comics, Taranee gets captured and tortured by Elyon and convinced that anything she sees is an illusion. When Will comes to retrieve her, Elyon tries to convince Taranee that rescuing Will is an illusion created by her too. But Will says that she's telling the truth and Taranee can read Will's mind if she wants to verify, and Taranee figures it out because Elyon never knew Taranee could read minds.
In Nightwing: Year One, Dick confronts Commissioner Gordon for the first time in his new guise as Nightwing. After Gordon prompts him to "say something only the kid would say" to prove his identity, Dick struggles for a bit before declaring "Holy Mistaken Identity!"
In The Return of Superman, Lois Lane asks one claimant to the name Superman why she should give him the time of day. His answer? "To Kill a Mockingbird," which Lane recognizes as the real Superman's all-time favorite movie.
In the Donald Duck story "Kappa! Kappa! Kappa!", Donald's nephews see what looks like a famous, ancient samurai show up to save kappa from the villainous tengu. They quickly realize the samurai is just Donald Duck in disguise... after they see him running away in panic in Donald's distinctive way.
In one early The Flash story (during Barry Allen's tenure in the costume, pre-Crisis), this nearly gets Barry in trouble when, while in costume and conversing with a crime scene investigator, he gives the guy a wink that only Barry (as a police scientist) would give him whenever in agreement about something. Naturally, the other cop starts putting two and two together, but Barry manages to convince him by the end of the story that he and Flash are definitely not the same person.
During Stephanie Brown's run as Batgirl, at one point Clayface forms into an exact copy of her to try to escape a bank robbery. After the two Batgirls shout the standard "Shoot her!" "No, shoot HER!", one Batgirl turns to the detective and declares. "Shoot me. I'll bleed." Detective Gage promptly shoots the other one because no one else would come up with such a stupid plan.
Confronted by a SWAT team, Mystique shapeshifted into a exact duplicate of the team leader; while the team were making their mind up over which one to shoot, one of the "leaders" shouted, "Shoot us both! I'm wearing body armour, she isn't!" The other shouted "No! No! Don't shoot!" The team immediately shoots both; both drop, only one gets up. A minute later one of the cops asks "him" why he's still standing, and "he" replies, "I moved all my vital organs into my lower body..."
In the My Little Pony: Friendship is Magic comic book, the Mane 6 are affirmed of the CMCs' identity because of their talk of getting their cutie marks from being fillynapped (cue deadpan "It's them" from Applejack).
Peter Parker!Spider-Man does this twice in the finale of Superior Spider-Man. The first time, he attempts to convince Spider-Man 2099 that he's the real deal and Doc Ock had control of his body. Miguel realizes that it was so stupid it makes sense and buddies up with him. The second time, he goes to confront the Green Goblin, who still thinks it's Octavius controlling Spidey. When the Goblin boasts that once the bombs go off, he'll have nothing left, Peter retorts "Except the dignity of knowing I never carried a man-purse", he realizes He's Back.
After Dooplissswaps bodies with Mario in chapter 43 of Paper Mario X 2, the real Mario is left alone with the pig who has been following the team — who is revealed to be Kirby. Kirby is relunctant of joining Mario, asking for proof that he's the real deal. Mario then brings up the ending of the first Paper Mario X to Kirby (triggering a flashback in the process). Kirby then claims that only the real Mario would remember their goodbye.
In Supernatural fanfic series "Sky Verse", Sam, Dean and Castiel realize they are vulnerable to impersonation and being tricked into an ambush by their enemies due to their use of cell phones to arrange to meet so they agree on passphrases they must use at the beginning of each phone call which are things they would never normally say and which their enemies would not be able to guess.
Eugenesis: Galvatron is trussed up in a room, and hears a person who sounds uncannily like Optimus Prime telling him to tell the Autobots what he knows, and then he will be let free. Galvatron eventually talks, and is released and told he can go. This is when he realises that is Optimus, because no other Autobot would ever let him go free. The problem is Optimus is supposed to be dead.
He also rolls an apple down his arm and throws it to her with his elbow; a trick he does as both Aladdin and Prince Ali. The animator who drew Jasmine's facial expression when she realizes this deserves an award.
Syndrome in The Incredibles tells Mr. Incredible "I am your biggest fan...", which makes Mr. Incredible realize Syndrome is actually the grown-up Buddy Pine.
In the Transformers movie, after Megatron's death, people are suddenly surprised by the arrival of a new, purple-and-black robot who crashes Starscream's coronation. Then he opens his mouth and smugly demeans the Decepticon's new leader before easily killing him.
Starscream: Who disrupts my coronation?!
Galvatron: Coronation, Starscream? This is bad comedy.
Galvatron: Here's a hint! (Transforms into a laser cannon and reduces Starscream to ash)
In Megamind, everyone calls the setting Metro City except for the title character, who pronounces it to rhyme with atrocity. When he's fighting Titan disguised as the superhero Metro Man in the climax, he accidentally gives himself away when he mentions the city's name. The only reason Titan knows this is because he heard Megamind say it dozens of times as a news cameraman.
Film — Live-Action
The phrase "As you wish" in The Princess Bride is the Former Trope Namer. Westley, Buttercup's love, has been presumed killed, but turns up as the Dread Pirate Roberts, and Buttercup recognizes him by his use of the phrase which, for him, meant "I love you." For bonus points, he gives her vague hints at first, and when she tells him "You can die too for all I care!" as she pushes him off a cliff, he shouts it as he rolls to his (apparent) doom. (This leads to a My God, What Have I Done? moment for Buttercup. She and Westley both get better.)
In the first X-Men movie, Wolverine has just dispatched the shapeshifting Mystique and is confronted by a suspicious Cyclops, leading to the page quote above. (This was one of just 2 or 3 lines that Joss Whedon wrote for the movie.)
Bruce/Batman: It's not who I am underneath, but what I do that defines me.
Bruce also reveals his identity to Gordon with a Call Back to Gordon attempting to comfort him after his parents died.
In Batman Returns, Bruce Wayne and Selina Kyle accidentally expose their identities as Batman and Catwoman to each other by unthinkingly reenacting a snippet of banter from their previous fight. In a slight variation, they switched lines in and out of costume.
Batman/Selina: Mistletoe can be deadly if you eat it.
Catwoman/Bruce: But a kiss can be even deadlier... if you mean it.
And this trope is also used in the 1989 Batman film and is how Bruce realizes the Joker is the thug who killed his parents. He repeats the phrase to Joker at the beginning of their final confrontation just before decking him.
Jack Napier/Joker: You ever dance with the devil in the pale moonlight?
Bane (whose mind has been overwritten by Smith reveals himself to Neo by addressing him as "Mr. Anderson" in his usual mocking tone three times. Neo is slow to catch on, if only because of how inconceivable it is to believe Smith is walking around in the real world using another man's body.
Happens again in the final battle when Smith says, "Everything that has a beginning has an end, Neo" — revealing that the Oracle is still in there somewhere.
From Ghost: the word "ditto", which Patrick Swayze's character would use to respond to "I love you", to the point where his widow refuses to believe it's him when he tells her those three words through a medium.
In The Italian Job, the villain realizes that he's on a date with the daughter of the man he killed years ago when she uses her father's trademark phrase, "I trust everyone; I just don't trust the devil inside them."
The movie Face/Off uses a physical gesture of affection in this way.
In Terminator 2: Judgment Day, the reprogrammed T-800 identifies itself as sent by the future John Connor to help by saying "come with me if you want to live," the first thing Kyle Reese said to Sarah Connor.
It's interesting that neither the T-1000 nor the T-X referred to a target by their first and last names, which is how machines in the series usually address people. They might have a brief glitch and do this, so it is a possibility that it could be explored if they do make another Terminator film.
In the James Bond movie The World Is Not Enough, oil heiress Elektra King and terrorist Renard give themselves away as conspirators by using an identical aphorism - 'There's no point in living if you can't feel alive'. A few minutes later, Renard jams his hand into Bond's injured shoulder—something else he could only have learned from Elektra, though when Bond confronts her about this, she points out that Bond has been wearing a sling and any number of Renard's spies could have told him about this.
Emil: (disbelieving) Wait a minute, I know you... you're dead. We killed you. We killed you! WE KILLED YOU!!
A version of this occurs in in Disney's Condorman, although it's not so much about identity as intent. Natalia and Woody (the titular hero) are sharing a moment together when he says the line, "I'll bring the dip if you bring the Dostoyevsky." Later, when it appears that she has changed her mind about defecting, she says it back to him, which prompts him to realize that she wants to be rescued after all.
In Spies Lies And Naked Thighs, a man is introduced as a secret agent who has been surgically altered to resemble an old friend of the main characters in order to stop an assassination attempt on two world leaders. Late in the film, one character who knows that the assassin is hiding nearby manages to tip him off with a signal from college which meant "There's a girl in the room", revealing that this agent is really the old friend.
Played with in The League of Extraordinary Gentlemen, after the Big Bad and The Mole have just attempted to destroy the Nautilus with planted explosives. The Reverse Mole, who has stowed away on the escape vessel with the bad guys, manages to send a message to the rest of the League with the coordinates of the escape route. They realize it's from him because of the way the message opens — he's the only one in the world who would address them collectively as "my freaky darlings."
Nickelodeon's original movie Rags, a Cinderella-esque story, has the protagonist (a Cinderfella) meet with the music star (equivalent of princess) at a masquerade ball, and on his way out he tells her, "Be you." Later, when she seeks his help to track down the mystery singer Rags, right before the auditions start he repeats his earlier line, inadvertently revealing himself and later saving himself from having his Secret Identity and big break stolen by his evil stepfather and stepbrother.
The protagonist in The Assignment, a Carlos the Jackal lookalike undercover as the real deal, proves his identity to his handler when his handler says "awfully warm for this time of year", to which he replies "but not as sticky as two summers ago". These are the two halves of a code phrase that was part of an assignment from long ago that his handler mentioned in conversation.
L.A. Confidential: when he's become the victim of a Have You Told Anyone Else?, Jack Vincennes says "Rolio Tomassi" as his dying words. That was the name Edmund Exley had just entrusted him with in a private conversation, and is a name he made up for his father's unknown killer, 'the guy who gets away with it'. When Dudley Smith gets nervous about who this "Rolio Tomassi" might be and what he might know, he starts asking around about him... to Exley, who realizes immediately that Dudley should have no way of knowing that name and confusing it with a real person unless he had something to do with Jack's murder.
Zartan from G.I. Joe: The Rise of Cobra is quite fond of whistling the tune to "Jolly Good Fellow". When the man we think is the President comes out of the bunker, he goes into his office, relaxes and begins whistling. No prizes for guessing the tune.
In Ransom, Tom Mullen knows for sure Jimmie Shaker is the mastermind when he hears him say "Shouldn't be a problem": a phrase he heard over and over when an FBI analyst was examining a recording of an earlier conversation between Tom and the kidnapper.
In The Sixth Sense, this is how Cole demonstrates his medium powers to his mother, by repeating several things that only his grandmother's ghost would know about.
The climax of Muppets Most Wanted involves Miss Piggy almost marrying Constantine, a master criminal impersonating Kermit. When the real Kermit crashes the wedding, Miss Piggy comes up with a way to distinguish them: she asks each in turn to marry her. The fake Kermit confidently says 'Yes, I will! Let's go right now!', while the real Kermit stutters awkwardly and tries to duck the issue.
In Eldest, the 2nd book of the Inheritance Cycle, Eragon recognizes the dragon rider in steel armor as his seemingly dead friend Murtagh when he spins his hand-and-a-half sword at his hip, much like he did when the two of them trained together.
"Jane Smith" was identified by her bizarre taunt ("Rub a monkey's tummy! Rub a monkey's tummy with your head!") in the children's book Wayside School Gets a Little Stranger.
In Harry Potter and the Deathly Hallows, the good guys use Polyjuice potion to create six decoy Harrys while traveling. However, Harry sees the face of someone he knows in the crowd attacking him. He attempts to disarm him, rather than stun or kill, and the Death Eaters realize that this one is him, as Harry, being a Technical Pacifist, also used Expelliarmus in his battles with them. In the film, it's Hedwig coming to the rescue which gives him away.
Then there's the passwords the Ministry suggested everyone set to identify impersonators. Mrs. Weasley's password to her husband was her husband's pet name for her; the embarrassing/adorable "Mollywobbles".
This was used time after time in Sweet Valley High, because of all the Twin Switches that were going on. In one example, the twins' friends prove which twin is which by asking Jessica to spell thief and asking Elizabeth who a certain soap star is. Jess spells it wrong and Elizabeth doesn't have a clue about the soap star. Somehow, their friends are convinced...
In Stone of Tears, the phrase "true as toasted toads" is used to positively identify a messenger as having come from Zedd.
Also subverted in the first book. Richard finds himself under the effects of an "Enemy Web", where people would perceive him as their enemy; Darken Rahl if they're Richard's ally, or as Richard if they were allied with Rahl. Richard sneaks into his brother's tent, gets his attention, and goes to do the the "Loser's Salute," which his brother forced him to do whenever he lost one of their play swordfights as kids, to identify himself. But his brother already recognizes him as Richard; this immediately tips Richard off that his brother was really in league with Rahl.
A bit later, Kahlan recognises Richard despite the Enemy web because she notices his habit of running a hand through his hair when frustrated.
In the last book, after using forbidden magic to save the world from some other forbidden magic (yeah, that happens a lot in this series), Richard believes that his wife's feelings prevented her from regaining her memories, which she lost as a result of the latter forbidden magic. He realizes she did regain her memories when she says, "You're a rare person, Richard," something she said about him during the first book.
In The Lord of the Rings, Aragorn quotes to Frodo a bit of poetry that Bilbo wrote about him. Note that Frodo knew the poem not from Bilbo, but because it was in a letter from Gandalf that he (Frodo) was reading right then. Aragorn did not know that the poem was in the letter.
Although Aragorn might have known or suspected that Gandalf would supply Frodo with that line, making it a reasonable thing to drop into the conversation.
In Body Parts by Michael Bates, the tip off is the way a girl says "difícil" just as the protagonist's friend used to before she went missing. What's it a tip off to? The fact that the girl was sewn together from body parts harvested from local kidnapped teens, including said friend, which (somehow) gives her an unconscious retention of their mannerisms. This is a bit different from usual, as the girl herself doesn't know the identities of her "donors".
In The Stars My Destination by Alfred Bester, Saul Dagenham recognizes a disguised Gully Foyle when he uses a pseudonym that Dagenham had implanted in Foyle's mind back when he first attempted to interrogate him via hypnosis.
In Kushiel's Mercy, even though Sidonie's under the influence of Fake Memories wiping out the memory of her boyfriend Imriel, and Imriel himself is a Manchurian Agent, the use of the word "always" when it comes up in conversation always triggers something for them, which they eventually figure out. (Note that "always" is NOT the trigger to unlock Imriel's true personality, but it seems to help the two blend together.)
In Unicorn Point, Bane accidentally reveals himself when he speaks a Phaze dialect in place of Mach's Proton-speak.
In Xanth there is a Sorceress who likes to possess people. Unfortunately, she often gives herself away by calling people "my pet". Also, the Demoness Metria is described as being easily recognizable, no matter what form she takes, because of her speech impediment (luckily, her alternate personalities have no such speech problems).
Played for laughs in the Culture novel Matter. When an Upper-Class Twit-turned-fugitive seeks help from his former tutor, the latter tests his identity through several questions, all of which he gets wrong. The tutor wearily concludes this is indeed his inept student of old.
Star Trek: New Frontier, "Treason": Si Cwan (a ghost) proves he's possessed his sister by fighting both Xyon and Calhoun (neither slouches in the fight department), beating Xyon and getting beat by Calhoun, but proving himself enough for Calhoun to call him "Cwan, you pompous ass!"
A variation in The Paths of the Dead: Tazendra (now a Lavode) is standing guard at Dzur Mountain when Piro and Kytraan arrive to see Sethra Lavode. Being Tazendra, she's ready to just jump down and attack them, but when she hears Piro use the exclamation "Cha!" she recognizes it not as Piro's Catch Phrase, but as his father's, and she's happy to escort her old friend's son in.
In the Dale Brown novel Flight of the Old Dog, Brad Elliot recognises a colonel at the Alaskan base the Old Dog crew are trying to get a tanker from and uses a few choice insults to identify himself.
In Mark Billingham's novel Lifeless, the man responsible for a string murders is rumbled by DI Thorne when, during a phone call, he repeats a phrase he used in an incriminating videotape which Thorne had seen earlier.
In the Gaunt's Ghosts novel Blood Pact, Tona identifies that she's from Gaunt with a message to Vaynom Blenner by reminding the latter that he lied about his father to Gaunt on the first day of scholam.
In the Elenium trilogy by David Eddings, Sparhawk and other members of the Pandion order of knights have a long-standing code to identify themselves to one another. When traveling in disguise, Sparhawk sends a messenger to the home of a brother Pandion with the message that he "has brought his little mother for a visit." Little mother is the Pandions' Term Of Endearment for Sephrenia, their tutor in magic. (She happens to be present for the dispatch, and is surprised to learn of the code in her honor.)
Little Pet Shop of Horrors, a book in the Bone Chillers series (similar to Goosebumps), is about an evil pet shop that turns children into animals and sells them as pets. The protagonist has a friend who can do somersaults. At the end of the book she gets a pet dog. When it stands up and does a somersault, she realizes who it is.
In the Dresden Files novel Ghost Story, Molly asks Harry (who's dead at the time) to prove it's not some supernatural nasty in disguise.
In general, this is used many times over the course of the series as the heroes must deal with enemies who can shapeshift into another person's form. Harry even pulled this on Mab because she was speaking through an intermediary, something she had not done previously. So, she invokes his debt (her control over him) and makes him feel incredible pain.
In the Liavek story, "A Well-Made Plan", a Body Swapped character shows up at his front door and tries to convince his butler that it's really him. The butler asks what the item that is the source of his magical power is, and the character explodes in fury, refusing to reveal it, particularly since he'd never told the butler in the first place. This is the response that the butler was looking for, and he gladly lets him in.
In the Discworld book The Truth, William recognizes the Big Badas his father by a phrase he commonly used, "A lie can run around the world before the truth has got its boots on."
Neatly inverted/played with in Ender’s Game. Ender, depressed and unwilling to keep playing the titular game, receives a letter from his sister Valentine, which is full of various jokes and references that only they share. He realizes that it was probably written by his "handlers" in order to cheer him up, because it's too full of these references. The real Valentine wouldn't have put in so many.
Inverted in Animorphs 'The Capture', when Jake is infested by a Yeerk. Ax can tell that by some indirect signs, but they become fully convinced, when Ax tries to touch Jake, and the Yeerk can't hide its standard Yeerk revulsion at an Andalite and insults him.
A slightly modified version, "something only she would type", is used in Paper Towns by John Green, where Quentin recognizes that a comment on the fictional Omnictionary website was written by the missing Margo, who has a habit of randomly capitalizing letters inSide worDs anD sentEnceS.
Live Action TV
In one of the most brutal subversions of this, in Star Trek: The Next Generation, the completely altered Jean-Luc Picard, aka Locutus, addresses the Enterprise crew after their failed attempt to destroy them. "Your resistance is hopeless... Number One." The look on Riker's face, and the crew's, was pure Oh Shit!!.
To clarify, The Reveal here is that Locutus—and therefore the Borg—had access to all Picard's memories, and therefore knew all the Federation's plans to try and defeat the Borg.
In one episode of Hannah Montana, Miley's cousin disguises as Hannah to reveal her secret to the world. Lily and Oliver stop her, but they can't tell the two apart. Oliver tells the girls if they want to kiss him. The fake one, who likes Oliver, agrees, while the real one says no.
In the episode "Dead Man's Party", Cordelia asks, "How do we know it's really you and not zombie Giles?", to which Giles responds, "Cordelia, do stop being tiresome," and Cordelia quips, "It's him."
In the fourth season, Faith attacks Buffy in her home and activates a magical device during the ensuing fight. Afterwards Joyce asks her daughter if she's all right, and "Buffy" answers with Faith's Catch Phrase ("I'm five by five"), letting the viewer know that a body switch has taken place. Later on in the episode, Buffy (in Faith's body) makes reference to the aforementioned episode of Giles-turned-Demon to prove to Giles it's really her.
But not before playing with the idea:
Buffy: Giles, it's me, and I can prove it. Ask me something only Buffy would know.
Giles: ...who's President?
Buffy: We're checking for Buffy, Giles, not a concussion.
Similarly, in the fourth season of Angel, Angel realises that Cordelia is possessed by the same power that is controlling the Beast when both use the endearment "my sweet".
In LOST, The Man in Black is inhabiting John Locke's body; when he meets Richard, he says to him: "Hello, Richard. It's good to see you out of those chains," recalling a conversation the two had centuries earlier when Richard was chained to the walls of the Black Rock.
In the second season, where Desmond is identified by his use of "brother."
Happened in the previous season with the whole Red Sox thing that Sawyer hears Jack say (Sawyer heard it before from Jack's father, that even mentioned that felt bad and wanted to call his son).
In "The Christmas Invasion" and the Children In Need mini-scene that leads up to it, the new Doctor demonstrates himself to be the same man as his previous incarnation by repeating to people the first thing he said when he met them in his previous body.
The episode "Midnight" had a strange "detect the enemy" variant on this: An alien being seems to possess a woman, and it keeps repeating whatever anyone says to it. The Doctor surmises this is its attempt at learning. Then it says things at exactly the same time as everyone else. Then it only does for it the Doctor. Then it speaks before the Doctor does... eventually, the woman seems back to normal, and the Doctor seems to be possessed. The woman convinces everyone else to throw the Doctor out the airlock... but two of the characters realize the being has just colonized the Doctor's mind and is still possessing the woman when she uses two phrases the Doctor used earlier.
"Turn Left" has the reappearance of "Bad Wolf" (series one's Arc Words) as a message from Rose to the Doctor, letting him know that holes must be opening between their parallel universes, which is very very bad.
River Song has her personal way of calling the Doctor for help: graffitiing a significant historical artifact with her current space-time coordinates, along with "Hello Sweetie."
In "The Enemy Of The World", the Second Doctor intends to prove to his companions that he's not the Identical Stranger villain Salamander by playing his recorder ... only he left it in the TARDIS, so he mimes playing it.
When the Doctor hears the phrase "Run, you clever boy. And remember" he knows he's found Oswin/Clara again. Twice.
The Outer Limits TOS episode "The Architects of Fear" features a poignant example. A man volunteers for a mission to be turned into an alien-like creature, climb inside a fake UFO and launch an "attack" against the earth. (All this as part of a plan by a shadowy organization to bring about world peace by uniting humanity against an external threat.) The plan goes awry however, when the "UFO" crashes and the grotesque "alien" is shot by hunters. The man's pregnant wife (who knows nothing about the mission) arrives on the crash scene and doesn't recognize the ship's alien-looking occupant as her husband—until he does a special pointing gesture that has special meaning for the two of them, just before he dies of his wounds.
In the episode "Crystal Skull", Daniel Jackson is shifted into another dimension that makes him invisible to everyone but his grandfather, who unfortunately is known to be slightly insane. Daniel instructs his grandfather to repeat some things after him to convince his team that he is standing right there with them. Daniel's frustration mounts as Jack refuses to believe his grandfather, to the point where he shouts, "Jack, don't be an ass!" which of course grandpa repeats word for word. Jack is taken aback and after a second he replies, "Daniel?"
The situation is reversed in the episode "Fragile Balance" where an infuriated "Daniel!" from a considerably younger version of Jack is taken as confirmation of his identity by Daniel (the two characters usually adopt a particular tone when speaking to each other).
The actor who played Young Jack should have gotten an award for the brilliant job he did of mimicking O'Neill's mannerisms and speech patterns.
The trope is lightly mocked in the episode "Spirits", where they encounter shapeshifting aliens:
Daniel: Jack. Are you you?
O'Neill: Yes. Are you?
O'Neill: Never mind.
Daniel: Don't shoot! Just let them tend to Xe'ls.
Jack: How do I know you're the real Daniel?
Jack: ... Yeah, okay. (lowers gun)
The second season episode "Holiday" also had a large dose of this. For example, when Jack and Teal'c have switched bodies:
Hammond: How did it go, colonel?
O'Neill: It did not go well, General Hammond.
Teal'c: Ya think?
The ancient communication stones used in Stargate Universe seem to be setting up for a whole swath of this trope. The stones allow faster than light communication through the simple expedient of swapping bodies with the host receiver at the other end. It kicked off in episode 9 with one of the crew being allowed to visit her girlfriend/lover back on Earth, but doing so in the guise of a totally different body. Asked if it was really her, the [blue eyed, Anglo] stranger replied in fluent Cantonese "Have you thrown away that ugly red chair yet?", which satisfied her lover as to her identity.
In Living Color! parodies this in its fake trailer for Ghost 2 . When Sammy Davis Jr.'s ghost tells the medium to relay to his wife he loves her, the wife says "Sammy would never say it like that". When faux-Whoopi tells her "Sh-Boing-Boing-Boooooiiiing", his wife instantly knows his spirit is there.
A character missing this was used in one episode of Family Matters, an episode that saw Steve Urkel bust a criminal and receive a death threat from his brother. This results in Carl and a fellow cop taking shifts in protecting Steve. For years, Carl had always said, "See you later," to that particular partner, who would always reply, "Not if I see you first." Later in the episode, when Carl left his watch, the "buddy" turned out to be the killer in disguise, but as he's about to kill Steve, Carl busts in with the police force and arrests him. As it turns out, the killer replied to Carl with a simple, "Yeah, see ya," letting Carl instantly know it wasn't him.
In an episode of Wings, Brian is caught in a bank robbery and hears the masked robber use the unusual phrase, "We'll all be sitting in butter!" Later, as he flies an unknown client back to the mainland, the client uses the same phrase, cluing Brian in to the fact that he's transporting the robber.
On Arrested Development, when Oscar has to convince Michael that he's not his twin brother George Senior:
Michael: No, no, don't buy it. I'm taking my son to the cabin, and there's nothing you can say to make me believe that you are not my father. Oscar: I understand. Your child comes first. Michael: Oh, my God, you're Oscar.
When Buster is pretending to be George Senior and refers to Lucille as "mother":
Buster: I mean lover. I love making love to mother. I mean lover.
Subverted on The Middle Man, when Wendy asks MM this and he refuses to divulge secrets through a civilian interpreter. That's good enough to prove it for Wendy.
Interesting variation in the final episode of Blackadder II. Blackadder has apparently sold out Queen Elizabeth to a master of disguise for a single key bit of information—in this case, Nursie's habit of dressing up as a cow at dress-up parties.
Blackadder: From this moment he was doomed. For you see, the prince is a master of disguise, while Nursie is an insane old woman with an udder fixation.
In "Turnabout Intruder", Kirk, while trapped in his ex-lover's body, tried to convince Spock that he was him. He mentioned certain events where Spock had helped him and urged him to help him again. Subverted when Spock calmly pointed out that those events could be easily read from the captain's logs and reports.
It's rather bizarrely subverted in "Whom Gods Destroy", where an insane former Starfleet officer develops the ability to morph into other people, and Spock is presented with two Kirks at the climax. He somehow can't come up with any questions that only Kirk would know, just so the episode can end with a fight scene. Leonard Nimoy himself was quite upset about it.
In the critically acclaimed episode "Beyond the Sea", the psychic death row inmate (although if his abilities are real or not are made deliberately ambiguous) freaks out Dana Scully a lot by referring to her by a nickname only her recently-deceased father calls her.
Also, in the two-part episode "Dreamland", in which Mulder switches bodies with an Area 51 worker. He tries to convince Scully that it's really him by rattling off some of her personal information. She is not impressed.
Mulder (as Morris): I'm Mulder. I'm really Mulder. I switched bodies, places, identities with this man Morris Fletcher the man that you think is Mulder, but he's not. Of course you don't believe me. Why was I expecting anything different? Your full name is Dana Katherine Scully. Your badge number is... Hell! I don't know your badge number. Your mother's name is Margaret. Your brother's name is Bill Jr. He's in the Navy and he hates me. Lately, for lunch, you've been having this six-ounce cup of yogurt, plain yogurt, into which you stir some bee pollen because you're on some kind of a bee pollen kick even though I tell you you're a scientist and you should know better.
Mulder (as Morris): Even that yogurt thing? That is so you. That is so Scully. Well, it's good to know you haven't changed. That's somewhat comforting.
A meta-example in Babylon 5: G'Kar has a vision of an angelic being (in the form of his late father) telling him that it's up to him and his fellow Narns to break the cycle of violence between themselves and the Centauri, or both sides will be destroyed. G'Kar asks "Who are you? Where have you been all this time?" The response comes: "I have always been here" — and that's when the audience realises the angelic being is actually Kosh.
In the Law & Order universe, to ascertain if someone is an undercover cop, the other cops ask what the color of the day is.
This was also used on NYPD Blue. The "color of the day" refers to a color that undercover officers wear to distinguish themselves from real criminals. If a bunch of cops raid a drug den, they might think twice before getting too rough with someone who's wearing, say, a green shirt if that's the color of the day. If you look at the blackboard behind Sipowicz's desk there's an entry for the color of the day, filled in each morning by Fancy (or whoever). Since NYPD Blue always strove for realism this is probably Truth in Television.
A storyline on Third Watch had the cops are searching for a pair of rapists who pose as police officers so that they can pull women over and then assault them. At the episodes end, as the fake cops are menacing a woman, the real cops arrive. The fake cops try to claim that the woman's screams for help are just the raving of someone high on drugs. Already suspicious (they know the woman and know she's not an addict), one of the real cops asks the fakers a question in "cop lingo". When he's unable to answer, they instantly know they've caught the criminals.
In Red Dwarf, Rimmer tries to discourage Lister from going ahead with his plan to become Rimmer's superior officer by posing as Kristine Kochanski (who Lister had a crush on and wants to revive as a hologram). Rimmer succeeds in convincing Lister that there's no point in doing it as "she" has no interest in him at all... until "she" says, "I want a man who's going places. Up, up, up the ziggurat, lickety-split!"
In "The Inquisitor", after Lister and Kryten are erased from the timeline to make way his "sperm-brother" and another mechanoid, Lister convinces their counterparts of their identities by listing several embarrassing personal details about Rimmer, that only Lister could possibly know.
When Haley has locked herself in the tutoring center during a school shooting on One Tree Hill, she identifies her husband when he completes their catchphrase through the door ("Always..." "...and forever.") and lets him in.
In the Young Blades episode "The Chameleon," after Captain Duval gets himself thrown in jail and the Musketeers believe he is a shapeshifter trying to frame Duval:
Siroc: If you're the real Captain Duval, tell us something only he would know.
Duval: Like what?
D'Artagnan: Like what we got you for your last birthday.
Duval: Nothing. Bunch of thoughtless, shiftless recruits.
In the The Sarah Jane Adventures episode "The Man Who Never Was", Luke twice sends a message to Rani and Clyde by unconventional means. Both times he proves it's him by calling them "Clani".
The Eureka episode "Jack of all Trades" has Jack swapping bodies with the Astreus's crew, starting with Fargo. At first Alison thinks Jack has gone crazy, and when "Fargo" corroberates, that he's playing along as some sort of joke. Then Jack tells Alison they were just making out, leading to this:
Fargo in Carter's body: Dr. Blake, PDA in the workplace is against GD policy...
Carter in Fargo's body: Bigger! Issues! Fargo!
Alison: Oh my God, you're not kidding.
When Jack shows up just after a Jack impostor tries to kill Zane, Jack's attempt to explain how he got there tips off Zane to the fact that he's telling the truth.
Zane: No computer simulation of you could be that incoherent.
An episode of Touched by an Angel had a high school teacher being accused of sexual harassment. The principal believed his vehement denials due to their longtime friendship and that the accuser was a known troublemaker. To that end, she obtained a copy of the girl's complaint. . .and was shocked to realize that the girl was telling the truth. How? The girl quoted, word for word, the same cheesy seduction lines that the man had used on her 25 years ago.
An episode of Cold Case had the detectives trying to deduce who planted a bomb that killed a married couple by interviewing the friends who had visited them the weekend before. It soon became obvious that the supposedly dead wife was in fact alive and pretending to be one of the friends in question (they bore a strong resemblance to each other)—it was she who died in the blast along with the husband—when one of the other friends mentioned hearing the wife say something very similar to what the woman had told them.
In an episode of 3rd Rock from the Sun, Dick gets locked in the stairway of a dentist's office because he is so desperate to smoke a cigarette. When Harry finally finds him, he shouts through the door:
Harry: Wait, how do I know it's really you..?
Dick: It's really me!
Harry: Well, that's something you would know!
Variant in Elementary: Watson asked someone who claimed to be looking for Sherlock "are you a friend of his?" "He doesn't have any friends" was the right answer.
An earlier episode also had an inversion. Holmes figures out who the killer-of-the-week is but ends up being kidnapped. Earlier, Watson is shown to be frustrated about Holmes texting like a "teenager on a sugar high" with tons of unrecognizable abbreviations and emoticons (Holmes believes it allows for efficiency in communication). When the kidnapper tries to tell Watson that everything is fine, she uses Holmes's phone but texts in a normal manner. Watson immediately goes to Captain Gregson. After he's freed, Holmes tries to get credit for "deliberately" getting the kidnapper to text Watson.
In an episode of Murdoch Mysteries a female witness uses the word "huckleberry" to describe anyone she does not consider to be an upstanding citizen. When Murchoch later reviews the transcript of an old trial he discovers that the key witness in that trial also used the word "huckleberry" to describe people she did not like. He quickly discovers that the two witnesses were actually the same woman who was paid by an unscrupulous prosecutor to assume fake identities and act as a prosecution witness so the prosecutor could win more cases.
In the pilot of Graceland, Mike is on his first serious undercover operation and is ordered by Russian mobsters to kill Donny, a DEA undercover agent who had his cover blown. They end up in a Mexican Standoff and Mike is trying to convince Donny of what is going on but Donny never met Mike and is highly suspicious of Mike's story. Mike tells him that he just moved into Donny's old room in "Graceland", the house the undercover agents share, and that Lauren, Donny's fellow DEA agent, told him not to touch any of Donny's stuff. Since this is exactly what Lauren would have told the New Meat, Donny believes Mike and they lower their guns.
The Once Upon a Time episode "The Thing You Love Most" has a variation: When Regina attempts to interrogate Mr. Gold about Emma coming to town, he asks her to "please" excuse him. This is the first hint to Regina and the viewers that he remembers his life as Rumpelstiltskin, since it recalls a conversation that they had in the Enchanted Forest part of his price to help her cast the curse is she must do whatever he asks when he adds "please" to the sentence.
In the Agents Of Shield season 1 finale, the message to Mike Peterson from his son to let him know he's been rescued includes something only they would say to prove it isn't a trick.
Mythology and Religion
It's believed that the Japanese custom of answering the phone with "moshimoshi?" was to affirm the speaker as human.
Older Than Feudalism: In The Bible, after Jesus comes Back from the Dead, not all of his disciples recognize him right away. On one occasion related by Luke, he walks and talks with two of them all the way from Jerusalem to Emmaus, and they don't realize who they are talking with until he prays before a meal — apparently, the prayer was something only he would say. (Then, as soon as they recognize him, he disappears.)
Based on the context, the words in question were most likely, "This is My Body."
Luke specifically says that Jesus gave thanks and broke the bread, which is how he described what happened before he said "This is my body" at the Last Supper, so a common interpretation is that the way Jesus broke the bread was distinctive.
In Metroid: Fusion, Samus (and the player) gets tipped off that the AI she named after her old commanding officer from her army days, Adam Malkovich, really is said officer when, after giving her an order, it asks, "Any objections, Lady?" - something her commanding officer always said after giving her a briefing.
That line was used again in the trailer for Metroid: Other M as a cue to the audience regarding what game Nintendo was announcing.
In Other M itself, Samus recognizes Anthony Higgs when he calls her "Princess" near the start of the game, and at the end.
Riku identifies Roxas as Sora's Nobody this way in Kingdom Hearts: 358/2 Days. After the two have a skirmish, he says "Come on Sora, I thought you were stronger than that." Without thinking, Roxas replies "Get real! Look which one of us is winning!", and immediately wonders why he said it, while Riku realizes that his opponent is "his Nobody".
In Digimon World 2, at some point late in the game, a member of the Blood Knights, Commander Damien, disguises himself as a Black Sword cadet. Your character gets suspicious, and in an attempt to draw him out, calls out "Commander Damien, sir!". Damien, of course, responds, but not long before he realizes he just blew his cover.
Quarians in Mass Effect are taught a certain phrase to use when returning to the Flotilla, as they usually arrive on a different ship than the one they left on. If they don't provide that phrase when prompted, then the Flotilla knows that they're returning under duress, and their ship is destroyed. They even have a separate phrase that sounds right if you're not in the know, but signals the Flotilla that something is wrong. It's also a strong signifier of the quarian sense of communal duty, since it also means that the ship they are on will be destroyed.
Another example occurs when, if you gave Tali the geth data during her personal mission in the first game, you can use it to prove it's really you when you meet her again in the second.
Played with in Super Paper Mario, where the player must determine who the real Merlee is in one chapter thanks to a shapeshifting villain. A game show is hosted, where the player can ask five questions, then choose afterwards. The real Merlee has a fly buzzing around her, since she spent half the chapter hiding in a toilet in a public restroom.
Swaine in Ni No Kuni has a habit of referring to his group of friends as "You lot"—"Stand back, you lot!" "Let's do this, you lot!" So when the party travels to the past, and the young Prince Gascon refers to them as "you lot," it immediately raises some suspicions...
In Dragon Quest VI, the chancellor of Somnia (who's effectively taken over the kingdom) uses this to get you thrown out - he asks the hero, who is being taken for the Prince of Somnia, the name of his late sister. None of the options you're given are correct, and he denounces you as an imposter. Subverted in that the hero is the Prince, but all his memories are tied up with his real body, which is on the other side of the continent right now.
In Dominic Deegan: Oracle for Hire, Gregory is disguised as "ace reporter Buzz Beardman"... but Rachel recognizes him right away when she hears the name, because Gregory created Buzz Beardman as a character in a comic book he made as a child.
Klo Tark's spirit identifies himself by mentioning "he who I would call my brother", his last words as a living person, to Dominic, the person he was referring to with said last words.
This same trait is later used in an inversion of this trope: Part of Tarquin's evidence that Elan is not Nale is that Elan did not ramble about his intellectual superiority, which Nale would have done.
In Sluggy Freelance, not so much Something Only They Would Say as Something Only They Would Do. Upon his return from an alternate dimension, Torg hugs Bun-Bun. This results in the inevitable beating. The other characters know that it's 'their' Torg, as only he would do something so stupid as use Bun-Bun's aggression to determine whether he's in the right dimension or not.
In thisxkcd strip, someone discovers that babbling enthusiastically about foolproof ways to establish his identity is apparently sufficient to establish his identity.
The Alt text makes it even better:
"Not sure why I just taught everyone to flawlessly impersonate me to pretty much anyone I know. Just remember to constantly bring up how cool it is that birds are dinosaurs and you'll be set."
In the Paradise setting, a number of people who have been involuntarily Changed into Funny Animals use this method to convince friends and loved ones that it's "really them".
In Red Panda Adventures, when the Flying Squirrel is hypnotized to believe that the enemy they're fighting is the Red Panda and the Red Panda is the bad guy, the Panda proves his identity by whispering "Kit Baxter, behave yourself!" in her ear.
In Johnny Test, Mr. Black and Mr. White make a robot clone of Johnny, and the others can't figure out which is the fake. Dukey figures it out when fake Johnny answers a math question very fast, since the real one always do the hands thing.
Inverted in X-Men; Morph, Back from the Dead and evil, has taken the form of Wolverine, and Jubilee finds them circling each other. One Wolverine shouts that she should shoot them both, and so naturally she shoots the other Wolverine - the real one. It's even funnier because if she had just blasted both of them, the real Wolverine would have been protected by his Healing Factor.
Elisa Maza of Gargoyles identifies Talon as her transformed brother Derek when he automatically answers her "cross my heart" with "hope to die", complete with the particular series of gestures the two of them customarily use with the phrase.
Again in Gargoyles, when Goliath is kidnapped by a shape shifter. When Elisa comes to save him, the shifter naturally takes her form. Since he and Elisa have been having trust issues the whole episode, Goliath tries to identify the real Eliza by shouting "Tell me the truth! Did you ever doubt me, even for a moment?" when Elisa sweetly replies, "Never!" Goliath knows that's the fake and tells his Minotaur cop-friend to shoot.
In Darkwing Duck, NegaDuck once dresses up as Darkwing to fool SHUSH. Then he runs into the real Darkwing in the presence of others, but confuses them by anticipating and copying his reactions and words exactly in real time... until the original suggests there's something cute around, triggering an immediate Axe CrazyBerserk Button reaction from his Evil Twin.
In one episode of Inspector Gadget, Gadget and an imposter disguised as him storm into an awards ceremony being held by the Chief and many other police officers. Both try to convince the Chief that the other is the imposter, and the real Gadget, while doing so, accidentally clobbers the Chief over the head with one of his gadgets. The Chief groans, and tells the officers to arrest the other one, saying that the one who hurt him is obviously the real Gadget.
Happens twice in Ben 10. When Ben turns into Heatblast for the first time, a horrified Gwen doesn't recognize him and is about to use a fire extinguisher on him when he says 'Don't even think about it, freak'. This is when she realizes it's Ben. The second time is in A Change of Face when Gwen and Charmcaster have exchanged bodies and the real Gwen must convince her family of her identity. The following conversation takes place:
Charmcaster (as Gwen): Who are you guys going to believe? This liar, or your own eyes?
Gwen (as Charmcaster): Ok, if you're Gwen, you should know the name of the teddy bear Ben sleeps with.
Ben: Hey! Furry Freddy has his own bed! It just...happens to be next to mine.
Gwen (as Charmcaster): (facepalming) You just gave her the answer, you dweeb! Did your parents send you to doofus school, or were you born like this?
Ben: (wide-eyed) Gwen?! It really IS you!
Done at least twice in Jackie Chan Adventures. The first time, Jackie is split into an overly nice half and an asshole half, and when asked which is the evil one, Nice-Jackie dissolves into tears and proclaims that he must be the evil one because he just killed a bug. Cue aside glances.
The second time played it a little straighter with normal-Jackie fighting an evil clone. The good guys want to help but can't tell them apart. Captain Black asks them when his birthday is, and is dismayed when neither of them know the answer. Jade figures it out pretty quickly.
Jade: Hey Jackie, will you take me to Moose World this weekend?
A variant of this trope appears in an episode of Goof Troop. Goofy and a double, who is a criminal, are in the same room, which Goofy had been wallpapering. Max identifies his father by leaving an open can of wallpaper glue in the middle of the floor and asking his dad to step forward. Both of them step forward, but only the real Goofy steps in the glue.
In the The Legend of Zelda cartoon episode "Doppleganger", Ganon kidnaps Zelda after using a magic mirror to create a dark-clothed evil Zelda clone to replace her. Late in the episode, the real Zelda has freed herself and both versions end up covered in mud to the point where it's impossible to tell them apart. Link proposes a kissing contest to identify the fake. He rates the first Zelda's kiss as "very good," but gets a smack in the face instead of a kiss from the second ("*sigh* That's my Zelda all right.")
In the Fairly Odd Parents, in the episode "Twistory", George Washington is supplanted by Benedict Arnold in disguise. Timmy discovers who is the true Washington by describing how good the quality of the wood of a coat rack is; the supplanter answers "So what?", but the real George Washington, who is obsessed with chopping anything made of wood, jumps and destroys the poor rack with an axe.
Founding Fathers: (all in deadpan) That's Washington.
In the Garfield and Friends episode "Binky Goes Bad", Garfield reveals the real Binky the Clown by getting a judge to say, "Order in the court!". The real Binky immediately responded with, "What's your order? Hamburgers and fries?", while the impostor stays silent. Garfield comments that the real Binky had never been able to resist a very old joke.
In Young Justice, the way little Billy convinces the team (but mostly Wally) that he really is Captain Marvel is by mentioning how he shouldn't have to bring Kid Flash nachos and pineapple juice to get on his good side all the time.
In the episode "Auld Acquaintances", Red Arrow is hiding from the mind-controlled Justice League when Aqualad finds him. To make sure he could trust Aqualad, he asks Aqualad "Who broke your heart?" To which Aqualad replies Tula... and Red Arrow himself, one of Aqualad's best friends, who was currently aiming an arrow at his chest the whole time.
Vanna: Steve! Rusty! It's me, Vanna! You've got to let us in. Steve: Hardy har har. Rusty: Vanna Banana's inside, little girl. Besides, if you were the real Vanna, you would know our names. Vanna: But I just called you Steve and Rusty. Rusty: Right. But I'M Rusty, and HE'S Steve.
Wild West COW Boys Of Moo Mesa: Marshall Moo Montana got a letter from "Masked Bull" challenging him. He figured it was actually written by Horribull because nobody else called him "Meatball Head".
In the season two finale of My Little Pony: Friendship Is Magic, Princess Cadence convinces Twilight Sparkle of her identity by reciting the song she and Twilight would sing when she foalsat her.
At the end of Batman: The Animated Series episode "Beware the Gray Ghost", Bruce Wayne tells Simon Trent exactly what he told him as Batman.
In the Code Lyoko episode "Xana's Kiss", the heroes are dealing with a polymorphic clone that keeps changing shape. When this turns into a case of Spot the Imposter where Odd has to tell the Clone from Yumi, he quickly determines which is the real Yumi when the Clone stays silent while the real Yumi calls him a "pea-brain". (Odd may be insulted a lot, but he knew that XANA would never call him "pea-brain".)
In the Danny Phantom episode "Splitting Images", Danny, while stuck in Poindexter's body in the Ghost Zone, contacts Sam and Tucker through the mirror in his locker to help him out. Since he looks like a green-eyed Poindexter, Sam tells him to prove his identity.
Danny: In Second Grade, Tucker threw up in your lunchbox, but he told you Ricky Marsh did it.
Sam: (small gasp. angry) What?! I kicked him off the monkey bars for that! (points to Tucker) It was you?! (gasp of realization)
Sam and Tucker: Danny?
At the end of the Teen Titans season 4 finale we get this bit of dialogue
Beast Boy: Okay, you're freaking me out here! The white robe and smile are weird enough, but...hugs?!? Are you still...you?
Raven: Blue is still my favorite color, and don't get used to the smile because you're still not funny.
Beast Boy: Huh? (embraces her) RAVEN!
Raven: (looking slightly irritated) Quit it!
In Regular Show, Rigby hires a Doug, a temp who eventually turns into Rigby. When Mordecai declares Doug the real Rigby, he asks for a hug. He then exposes that he knows Doug isn't really Rigby because he knows Rigby would never hug him.
On Gravity Falls, as Dipper and his friends are about to go into a secret bunker they discovered, he tells them that they have to keep whatever they find a secret; Wendy responds by making a zipping motion on her lips. Later, when they come across a Shapeshifting monster, it takes Wendy's form and the two wrestle on the ground. Dipper yells for the real Wendy to give him a sign that it's her; one of them winks, but the other makes the zipping motion.
In one episode of Xiaolin Showdown Omi has to watch Dojo, who is a risk due to a problem that occurs every few thousand years. Eventually, Dojo manages to fool him and escapes. Leaving Omi in the cage. When the other monks return...
Clay: Wait, how do we know that's Omi?
Raimundo: Omi wouldn't leave his post.
Omi(very angry) I am AT my post! Actually I am inside my post! That confounded Dojo has pulled the carpet over my eyes!
The entire concept of the shibboleth, a way of identifying someone as an ally as described in the Book of Judges: where captured Ephraimites are betrayed by the fact that they can't pronunce the "sh" phoneme and would say "sibboleth" instead, is based around this. These days, shibboleths are call-and-response; a belligerent in a war will have a certain phrase and counter-phrase. If an approaching battalion gets the phrase wrong, they're the enemy. If the guard gets the counter-phrase wrong, the outpost is under enemy control. Notable examples include:
During the invasion of Normandy, the Americans would open with "Flash," counter with "Thunder," and confirm with "Welcome." This hinges on the fact that a German would pronounce it "Velcome," and caused problems for some German paratroopers that defected, as well as some German- and Yiddish-speaking Jews who had fled German rule and joined the US Army.
American POWs in Vietnam communicated by tapping Morse code on their cell walls. One would initiate a conversation by tapping the familiar "shave and a haircut" pattern, and the other would finish with "two bits". If the respondent instead repeated "shave and a haircut", it meant he was a Vietnamese agent.
English rioters during the Peasants' Revolt went around looking for Flemish weavers to beat up, identifying them by demanding people say "bread and cheese", which the Flemings did with a distinctive accent.
About eighty ears previously, the Flemish used "Schild en Vriend" (shield and friend), which the French who they were revolting against could not pronounce, making it Scilt, so the Flemish knew who to butcher.
Sectarian fighters in Northern Ireland can supposedly identify whether someone is a Catholic or a Protestant depending on how they pronounce the letter H.
Shibboleths need not be confined to wartime or otherwise violent situations. A lot of Swiss people have a habit of making any and all foreigners say 'Chochichäschtli' (a small kitchen cupboard). Not only is it rather difficult to get all these 'ch' sounds right, even if people do, they usually have trouble pronouncing the vowels the correct way. However, if someone is Welsh or Scottish, they usually manage pretty well.
If you're in New York City and don't want to be identified as a tourist, don't pronounce Houston Street like the city in Texas. Pronounce it like "HOUSE-ten" - the street name is a corruption of William Houstoun, who represented Georgia at both the Continental Congress and the US Constitutional Convention (the street used to be part of his father-in-law's estate).
FBI turncoat Robert Hanssen was caught this way. After several US agents had been captured and executed by the Soviets, the Bureau knew they had a mole on their hands. No one suspected Hanssen, however; he was even tasked with finding the leaks for a time. Eventually a communique between Hanssen and his handlers was intercepted, but there was no way to tell who the mole was—until an agent recognized a racial slur the mole made as something only Hanssen would say.
Former dictator of the Dominican Republic Rafael Trujillo decided that he didn't want any Haitians in his country (the Dominican Republic and Haiti share the island of Hispaniola). So, he had his agents show parsley to people near the border and ask them what it was. The Spanish speaking Dominicans would be able to pronounce the Spanish word 'perejil' ('parsley') properly, while French speaking Haitians would not. Those who couldn't were usually killed and it's estimated that there were between 20,000 and 30,000 deaths in the "Parsley Massacre".
A Finnish shibboleth during the WWII was Höyryjyrä (literally "steamroller"). The voiced 'h', frontal 'ö' and 'ä' phonems, labial 'y' and rolling 'r' are almost impossible for anyone except native Finn to pronounce.
Likewise, the name of the town of Jyväskylä would be nigh impossible for anyone but a native Finn to pronounce.