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* In ''Literature/AWindInTheDoor'', Meg is tasked with figuring out which of 3 identical men is the real Mr. Jenkins. She asks them all how they plan to help Charles Wallace at school and realizes [[spoiler:the real Mr. Jenkins is the one who is annoyed by the test and doesn't get what's going on, instead of spouting platitudes about what the school will do.]]
to:
* In ''Literature/AWindInTheDoor'', Meg is tasked with figuring out which of 3 identical men is the real Mr. Jenkins. She asks them all how they plan to help Charles Wallace at school and realizes [[spoiler:the real Mr. Jenkins is the one who is annoyed by the test and doesn't get what's going on, instead of spouting platitudes about what the school will do.]]]]
* ''Literature/TheChroniclesOfDorsa'': To test if the Tasia whom she sees is a dream simulacrum or the real one, Joslyn questions her about how she'd confessed to loving her. She passes, revealing to Joslyn it's the real Tasia.
* ''Literature/TheChroniclesOfDorsa'': To test if the Tasia whom she sees is a dream simulacrum or the real one, Joslyn questions her about how she'd confessed to loving her. She passes, revealing to Joslyn it's the real Tasia.
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* In the Literature/{{Discworld}} book ''Literature/TheTruth'', William recognizes the BigBad [[spoiler:as his father]] by a [[ArcWords phrase]] he commonly used, "A lie can run around the world before the truth has got its boots on."
to:
* ''Literature/{{Discworld}}'':
** Inthe Literature/{{Discworld}} book ''Literature/TheTruth'', William recognizes the BigBad [[spoiler:as his father]] by a [[ArcWords phrase]] he commonly used, "A lie can run around the world before the truth has got its boots on.""
** In ''Literature/TheScienceOfDiscworld II: The Globe'', this is how Ridcully proposes the wizards prove to their past selves that they're really them. The Dean is uncomfortable with this plan, since wizards often have secrets they wouldn't want to tell to themselves. (The one he uses involves a secret hiding place in his study.)
** In
** In ''Literature/TheScienceOfDiscworld II: The Globe'', this is how Ridcully proposes the wizards prove to their past selves that they're really them. The Dean is uncomfortable with this plan, since wizards often have secrets they wouldn't want to tell to themselves. (The one he uses involves a secret hiding place in his study.)
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* In the light novel ''The Disappearance of LightNovel/HaruhiSuzumiya'' (volume 4), [[spoiler:Kyon uses [[TrustPassword an alias known only to him and Haruhi]] to get bizarro-Haruhi to listen to and believe his words, and makes a mental note to keep it as an emergency measure from then on.]]
to:
* In the light novel ''The Disappearance of LightNovel/HaruhiSuzumiya'' (volume 4), Literature/HaruhiSuzumiya'', [[spoiler:Kyon uses [[TrustPassword an alias known only to him and Haruhi]] to get bizarro-Haruhi to listen to and believe his words, and makes a mental note to keep it as an emergency measure from then on.]]
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* ''Literature/TwentySixSixtySix'': Lotte realizes that [[spoiler: Archimboldi is really her brother Hans, based upon the descriptions she read in one of his novels.]]
* In ''Literature/ABrothersPrice'', Jerin's sister Blush was outside, and when she returns, cannot remember the password. This causes her to use some colourful curses, so that they already know it can only be her when she finally remembers the password. (They all learned how to curse from their grandmothers. Other families in the area probably don't have quite the same vocabulary...)
* In ''Eldest'', the 2nd book of the Literature/InheritanceCycle, Eragon recognizes the dragon rider in steel armor as his [[spoiler: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 ''Literature/WaysideSchool Gets a Little Stranger''.
* In ''Literature/HarryPotterAndTheDeathlyHallows'', the good guys [[spoiler: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; Harry, being a TechnicalPacifist, also used Expelliarmus in his battles with them]]. (In the film, it's Hedwig coming to the rescue which gives him away.)
** In the same book, Voldemort [[SpeakOfTheDevil jinxes his own name so that whoever says it is immediately found]]. The only people brave enough to do that, rather than referring to him by euphemisms or "the Dark Lord", are the ones leading the fight against him.
** 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".
* In the light novel ''The Disappearance of LightNovel/HaruhiSuzumiya'' (volume 4), [[spoiler:Kyon uses [[TrustPassword an alias known only to him and Haruhi]] to get bizarro-Haruhi to listen to and believe his words, and makes a mental note to keep it as an emergency measure from then on.]]
* ''Literature/Mask2020'': Side-Splitter makes an announcement over the radio about sinking every battleship on the sea. His finishes off his broadcast by saying "We'll stop at nothing, even when it's raining gnats and frogs.". This causes Akiko to freeze up in horror, because that's the phrase her mom likes to use instead of "it's raining cats and dogs". This confirms in her mind the suspicions that her mom is working with The Doll Lady and Side-Splitter. [[spoiler:Thankfully, her mom actually works for Room 12.]]
* This was used time after time in ''Literature/SweetValleyHigh'', because of all the {{Twin Switch}}es 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 another book, Enid (Elizabeth's best friend) and Todd (her boyfriend) are convinced that Elizabeth has told another guy everything about them when the guy repeats to them things they told her. When he tries the same thing with Jessica, she calls his bluff and later realizes that he stole Elizabeth's diary.
* In ''[[Literature/SwordOfTruth 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; [[BigBad 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 "Loser's Salute," which his brother forced him to do whenever he lost one of their play swordfights as kids, to identify himself. [[spoiler: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, Zedd recognises Richard despite the Enemy web because she notices his [[CharacterTics habit]] of running a hand through his hair when frustrated.
** In the last book, [[spoiler:after using forbidden magic to save the world from some other forbidden magic ([[TheWorldIsAlwaysDoomed 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 ''Literature/TheLordOfTheRings'', Aragorn quotes to Frodo a bit of poetry that Bilbo wrote about him. 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 he might have known or suspected that Gandalf would supply Frodo with that line, making it a reasonable thing to drop into the conversation.
** In the third book, Sam gives Frodo the password "Elbereth" to lower a ladder in the Tower of Cirith Ungol, reasoning that "No orc would say that".
* 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 ''Literature/TheStarsMyDestination'' 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 ''[[Literature/KushielsLegacy Kushiel's Mercy]]'', even though Sidonie's under the influence of FakeMemories wiping out the memory of her boyfriend Imriel, and Imriel himself is a ManchurianAgent, the use of the word "always" when it comes up in conversation always triggers something for them, which they eventually figure out. ("Always" is NOT the trigger to unlock Imriel's true personality, but it seems to help the two blend together.)
* In ''[[Literature/ApprenticeAdept Unicorn Point]]'', Bane accidentally reveals himself when he speaks a Phaze dialect in place of Mach's Proton-speak.
* In ''Literature/{{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 [[SplitPersonality alternate personalities]] have no such speech problems).
* Played for laughs in the [[Literature/TheCulture Culture]] novel ''Literature/{{Matter}}''. When an UpperClassTwit-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.
* ''Literature/StarTrekNewFrontier'', "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 ''Literature/ThePathsOfTheDead'': Tazendra (now a Lavode) is standing guard at Dzur Mountain when Piro and Kytraan arrive to see Sethra Lavode. Being [[BloodKnight 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 CatchPhrase, but as his ''father's'', and she's happy to escort her old friend's son in.
* In the Creator/DaleBrown 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 [[spoiler: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 ''Literature/GauntsGhosts'' novel ''Blood Pact'', Tona identifies that she's from Gaunt with a message to [[spoiler:Vaynom Blenner]] by reminding the latter that he lied about his father to Gaunt on the first day of scholam.
* In ''Literature/TheElenium'' trilogy by Creator/DavidEddings, 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'' from the ''Literature/BoneChillers'' series is about an evil pet shop that turns children into animals and sells them as pets. [[spoiler: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.]]
* ''Literature/TheDresdenFiles'':
** In the novel ''Ghost Story'', Molly asks Harry ([[spoiler:who's dead at the time]]) to prove he's not some supernatural nasty in disguise.
--->'''Harry:''' [[Franchise/StarWars You will go to the Dagobah System.]] There you will learn from Yoda, [[TearJerker the Jedi Master who instructed me.]]
** 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 [[spoiler: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 ''Literature/{{Liavek}}'' story, "A Well-Made Plan", a FreakyFridayFlip-ed 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 Literature/{{Discworld}} book ''Literature/TheTruth'', William recognizes the BigBad [[spoiler:as his father]] by a [[ArcWords phrase]] he commonly used, "A lie can run around the world before the truth has got its boots on."
* {{Inverted|Trope}} in ''Literature/EndersGame''. Ender, depressed and unwilling to continue his training, receives a letter from his sister Valentine, which is full of various jokes and references that only they share. He realizes that [[spoiler:she wrote it at the behest of his "handlers" in order to cheer him up]], because it's ''too'' full of these references.
* ''Literature/{{Animorphs}}'': After SixthRangerTraitor David acquires Marco's DNA, the others ask the real Marco BluffTheImpostor-type questions. He proves himself by giving characteristically [[DeadpanSnarker smart-ass]] answers.
-->'''Marco:''' Now can you all stop playing that game? I’m afraid I’ll miss a punchline and [[TheBigGuy Rachel]] will morph to [[BearsAreBadNews grizzly]] and eat me before I have a chance to say anything.
* A slightly modified version, "something only she would type", is used in ''Paper Towns'' by John Green. 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.
* An interesting variant in ''Literature/EmilyTheStrangeStrangerAndStranger''. To determine which Emily is the right one, a game of Emily Jeopardy was created to determine who was the real Emily. The two Emilys came up with questions for the other.
* In ''[[Literature/WingsOfFire Moon Rising]]'', Moonwatcher's telepathic mentor [[note]]being trapped far away from civilization[[/note]] asks her to read a particular history book. It details Fathom's execution of Darkstalker, which was achieved through an enchanted artifact.
-->'''Moon:''' I wonder what he enchanted.\\
'''Her mentor:''' It was a bracelet.
* In the ''Franchise/StarWarsLegends'' novel ''[[Literature/HandOfThrawn Vision of the Future]]'', Han Solo arranged to meet Lando secretly. Knowing that Imperial forces may be monitoring his communications, he instructed Lando to meet him "Ten systems Coreward from the place where you had no choice." In other words, ten systems Coreward from Bespin, [[Film/TheEmpireStrikesBack where Lando "had no choice" but to betray Han and his allies.]]
* In the ''Franchise/{{Mistborn}}'' books there is a race of intelligent beings called Kandra who can use a persons (or animals) bones in order to perfectly imitate that person's appearance. They gain none of their memories, but will typically observe or interrogate the person before taking over in order to more effectively imitate them. Determining who has been killed and replaced with a Kandra is a major plot point in several books in the series.
** In ''Literature/MistbornTheOriginalTrilogy'' Vin discovers a Kandra has infiltrated the group and spends a good deal of the second book trying to find them with the help of an allied Kandra named [=OreSeur=]. She asks various members of the crew about specific details of private events in hopes discovering the spy, but in the end [[spoiler:Vin fails to find the spy ahead of time because the enemy Kandra, [=TenSoon=], had actually replaced [=OreSeur=], not one of the other humans.]]
** In the second Mistborn era book ''Literature/ShadowsOfSelf'' a rogue Kandra is causing havoc throughout the city and attempts to assassinate the Governor. The main characters all set up unique call and response pass-phrases with each other in order to prevent the Kandra from replacing one of them, however [[spoiler:the Kandra had already replaced the Governor at some point before the book started, making the phrases useless.]]
* In the "Whoops, I shouldn't have said that" form, this trope is used in Creator/RobertGoldsborough's Literature/NeroWolfe novel ''Murder In E-Minor''. Two people both use the same slightly-odd phrase to describe the state of their relationship, which tips Wolfe off that they've rehearsed their stories together.
* In ''Literature/ACollegeOfMagics'', [[spoiler:Tyrian is apparently killed in the final confrontation, but due to the high levels of magic rampaging about is still alive but occupying the body of one of the bad guys. This trope is how he persuades Faris it's him in there]].
* While any human can speak the [[TrustPassword Master-Word]] and prove they're human, only the narrator/hero of [[Literature/TheNightLand The Night Land]] knows that Naani is the reincarnation of Mirdath, and only he knows that name or the story of their love in that previous life so many millions of years ago. He describes what he remembers of it to her to prove that he is the reincarnation of her lost love. (And that she's not crazy for remembering it.)
* In the backstory of ''Literature/TheWitchlands'', Ragnar decided to check if a monk famous for being able to track down anything was his missing son, a ScarilyCompetentTracker. To do that, he sent out a letter asking to help find a missing dog called Boots (the name of the boy's beloved childhood pet), figuring that someone unfamiliar with the name would refuse such trivial request. It worked.
* In ''Literature/AWindInTheDoor'', Meg is tasked with figuring out which of 3 identical men is the real Mr. Jenkins. She asks them all how they plan to help Charles Wallace at school and realizes [[spoiler:the real Mr. Jenkins is the one who is annoyed by the test and doesn't get what's going on, instead of spouting platitudes about what the school will do.]]
* In ''Literature/ABrothersPrice'', Jerin's sister Blush was outside, and when she returns, cannot remember the password. This causes her to use some colourful curses, so that they already know it can only be her when she finally remembers the password. (They all learned how to curse from their grandmothers. Other families in the area probably don't have quite the same vocabulary...)
* In ''Eldest'', the 2nd book of the Literature/InheritanceCycle, Eragon recognizes the dragon rider in steel armor as his [[spoiler: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 ''Literature/WaysideSchool Gets a Little Stranger''.
* In ''Literature/HarryPotterAndTheDeathlyHallows'', the good guys [[spoiler: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; Harry, being a TechnicalPacifist, also used Expelliarmus in his battles with them]]. (In the film, it's Hedwig coming to the rescue which gives him away.)
** In the same book, Voldemort [[SpeakOfTheDevil jinxes his own name so that whoever says it is immediately found]]. The only people brave enough to do that, rather than referring to him by euphemisms or "the Dark Lord", are the ones leading the fight against him.
** 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".
* In the light novel ''The Disappearance of LightNovel/HaruhiSuzumiya'' (volume 4), [[spoiler:Kyon uses [[TrustPassword an alias known only to him and Haruhi]] to get bizarro-Haruhi to listen to and believe his words, and makes a mental note to keep it as an emergency measure from then on.]]
* ''Literature/Mask2020'': Side-Splitter makes an announcement over the radio about sinking every battleship on the sea. His finishes off his broadcast by saying "We'll stop at nothing, even when it's raining gnats and frogs.". This causes Akiko to freeze up in horror, because that's the phrase her mom likes to use instead of "it's raining cats and dogs". This confirms in her mind the suspicions that her mom is working with The Doll Lady and Side-Splitter. [[spoiler:Thankfully, her mom actually works for Room 12.]]
* This was used time after time in ''Literature/SweetValleyHigh'', because of all the {{Twin Switch}}es 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 another book, Enid (Elizabeth's best friend) and Todd (her boyfriend) are convinced that Elizabeth has told another guy everything about them when the guy repeats to them things they told her. When he tries the same thing with Jessica, she calls his bluff and later realizes that he stole Elizabeth's diary.
* In ''[[Literature/SwordOfTruth 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; [[BigBad 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 "Loser's Salute," which his brother forced him to do whenever he lost one of their play swordfights as kids, to identify himself. [[spoiler: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, Zedd recognises Richard despite the Enemy web because she notices his [[CharacterTics habit]] of running a hand through his hair when frustrated.
** In the last book, [[spoiler:after using forbidden magic to save the world from some other forbidden magic ([[TheWorldIsAlwaysDoomed 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 ''Literature/TheLordOfTheRings'', Aragorn quotes to Frodo a bit of poetry that Bilbo wrote about him. 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 he might have known or suspected that Gandalf would supply Frodo with that line, making it a reasonable thing to drop into the conversation.
** In the third book, Sam gives Frodo the password "Elbereth" to lower a ladder in the Tower of Cirith Ungol, reasoning that "No orc would say that".
* 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 ''Literature/TheStarsMyDestination'' 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 ''[[Literature/KushielsLegacy Kushiel's Mercy]]'', even though Sidonie's under the influence of FakeMemories wiping out the memory of her boyfriend Imriel, and Imriel himself is a ManchurianAgent, the use of the word "always" when it comes up in conversation always triggers something for them, which they eventually figure out. ("Always" is NOT the trigger to unlock Imriel's true personality, but it seems to help the two blend together.)
* In ''[[Literature/ApprenticeAdept Unicorn Point]]'', Bane accidentally reveals himself when he speaks a Phaze dialect in place of Mach's Proton-speak.
* In ''Literature/{{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 [[SplitPersonality alternate personalities]] have no such speech problems).
* Played for laughs in the [[Literature/TheCulture Culture]] novel ''Literature/{{Matter}}''. When an UpperClassTwit-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.
* ''Literature/StarTrekNewFrontier'', "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 ''Literature/ThePathsOfTheDead'': Tazendra (now a Lavode) is standing guard at Dzur Mountain when Piro and Kytraan arrive to see Sethra Lavode. Being [[BloodKnight 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 CatchPhrase, but as his ''father's'', and she's happy to escort her old friend's son in.
* In the Creator/DaleBrown 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 [[spoiler: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 ''Literature/GauntsGhosts'' novel ''Blood Pact'', Tona identifies that she's from Gaunt with a message to [[spoiler:Vaynom Blenner]] by reminding the latter that he lied about his father to Gaunt on the first day of scholam.
* In ''Literature/TheElenium'' trilogy by Creator/DavidEddings, 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'' from the ''Literature/BoneChillers'' series is about an evil pet shop that turns children into animals and sells them as pets. [[spoiler: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.]]
* ''Literature/TheDresdenFiles'':
** In the novel ''Ghost Story'', Molly asks Harry ([[spoiler:who's dead at the time]]) to prove he's not some supernatural nasty in disguise.
--->'''Harry:''' [[Franchise/StarWars You will go to the Dagobah System.]] There you will learn from Yoda, [[TearJerker the Jedi Master who instructed me.]]
** 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 [[spoiler: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 ''Literature/{{Liavek}}'' story, "A Well-Made Plan", a FreakyFridayFlip-ed 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 Literature/{{Discworld}} book ''Literature/TheTruth'', William recognizes the BigBad [[spoiler:as his father]] by a [[ArcWords phrase]] he commonly used, "A lie can run around the world before the truth has got its boots on."
* {{Inverted|Trope}} in ''Literature/EndersGame''. Ender, depressed and unwilling to continue his training, receives a letter from his sister Valentine, which is full of various jokes and references that only they share. He realizes that [[spoiler:she wrote it at the behest of his "handlers" in order to cheer him up]], because it's ''too'' full of these references.
* ''Literature/{{Animorphs}}'': After SixthRangerTraitor David acquires Marco's DNA, the others ask the real Marco BluffTheImpostor-type questions. He proves himself by giving characteristically [[DeadpanSnarker smart-ass]] answers.
-->'''Marco:''' Now can you all stop playing that game? I’m afraid I’ll miss a punchline and [[TheBigGuy Rachel]] will morph to [[BearsAreBadNews grizzly]] and eat me before I have a chance to say anything.
* A slightly modified version, "something only she would type", is used in ''Paper Towns'' by John Green. 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.
* An interesting variant in ''Literature/EmilyTheStrangeStrangerAndStranger''. To determine which Emily is the right one, a game of Emily Jeopardy was created to determine who was the real Emily. The two Emilys came up with questions for the other.
* In ''[[Literature/WingsOfFire Moon Rising]]'', Moonwatcher's telepathic mentor [[note]]being trapped far away from civilization[[/note]] asks her to read a particular history book. It details Fathom's execution of Darkstalker, which was achieved through an enchanted artifact.
-->'''Moon:''' I wonder what he enchanted.\\
'''Her mentor:''' It was a bracelet.
* In the ''Franchise/StarWarsLegends'' novel ''[[Literature/HandOfThrawn Vision of the Future]]'', Han Solo arranged to meet Lando secretly. Knowing that Imperial forces may be monitoring his communications, he instructed Lando to meet him "Ten systems Coreward from the place where you had no choice." In other words, ten systems Coreward from Bespin, [[Film/TheEmpireStrikesBack where Lando "had no choice" but to betray Han and his allies.]]
* In the ''Franchise/{{Mistborn}}'' books there is a race of intelligent beings called Kandra who can use a persons (or animals) bones in order to perfectly imitate that person's appearance. They gain none of their memories, but will typically observe or interrogate the person before taking over in order to more effectively imitate them. Determining who has been killed and replaced with a Kandra is a major plot point in several books in the series.
** In ''Literature/MistbornTheOriginalTrilogy'' Vin discovers a Kandra has infiltrated the group and spends a good deal of the second book trying to find them with the help of an allied Kandra named [=OreSeur=]. She asks various members of the crew about specific details of private events in hopes discovering the spy, but in the end [[spoiler:Vin fails to find the spy ahead of time because the enemy Kandra, [=TenSoon=], had actually replaced [=OreSeur=], not one of the other humans.]]
** In the second Mistborn era book ''Literature/ShadowsOfSelf'' a rogue Kandra is causing havoc throughout the city and attempts to assassinate the Governor. The main characters all set up unique call and response pass-phrases with each other in order to prevent the Kandra from replacing one of them, however [[spoiler:the Kandra had already replaced the Governor at some point before the book started, making the phrases useless.]]
* In the "Whoops, I shouldn't have said that" form, this trope is used in Creator/RobertGoldsborough's Literature/NeroWolfe novel ''Murder In E-Minor''. Two people both use the same slightly-odd phrase to describe the state of their relationship, which tips Wolfe off that they've rehearsed their stories together.
* In ''Literature/ACollegeOfMagics'', [[spoiler:Tyrian is apparently killed in the final confrontation, but due to the high levels of magic rampaging about is still alive but occupying the body of one of the bad guys. This trope is how he persuades Faris it's him in there]].
* While any human can speak the [[TrustPassword Master-Word]] and prove they're human, only the narrator/hero of [[Literature/TheNightLand The Night Land]] knows that Naani is the reincarnation of Mirdath, and only he knows that name or the story of their love in that previous life so many millions of years ago. He describes what he remembers of it to her to prove that he is the reincarnation of her lost love. (And that she's not crazy for remembering it.)
* In the backstory of ''Literature/TheWitchlands'', Ragnar decided to check if a monk famous for being able to track down anything was his missing son, a ScarilyCompetentTracker. To do that, he sent out a letter asking to help find a missing dog called Boots (the name of the boy's beloved childhood pet), figuring that someone unfamiliar with the name would refuse such trivial request. It worked.
* In ''Literature/AWindInTheDoor'', Meg is tasked with figuring out which of 3 identical men is the real Mr. Jenkins. She asks them all how they plan to help Charles Wallace at school and realizes [[spoiler:the real Mr. Jenkins is the one who is annoyed by the test and doesn't get what's going on, instead of spouting platitudes about what the school will do.]]
