The cast has been infiltrated by a shapeshifter, replicant, robot, an evil-doer or outcast who has posed as one of them, or some other creature that is able to pass as human; or else someone formerly trustworthy has lost their humanity to The Virus or some other alien parasite and is now secretly working against them. How do the other characters determine which of them is no longer human?
If they already know that the impostor has a certain type of Glamour Failure or Kryptonite Factor, then they can use that weakness as the basis of an Impostor Exposing Test. Or they might decide to Pull the Thread.
If the impostor is an alien, they can cut themselves and see who has Alien Blood. If it's a vampire, they can dip their hands in holy water and see who gets burned. If it's someone impersonating a famous figure, perhaps ask them a question about some little known detail of that figure's life. If the person is purportedly a close friend or family member, you can ask them something secret and private that only the real person knows ("What flavor milkshake did we have on our honeymoon?").
This can go down in a number of ways. Someone accused of being an impostor may simply perform the test on themself to prove that they're human. More dramatically, there may be a high-tension scene where all the suspects gather together and perform the test one by one. When the impostor is exposed by the test, or when its turn to take the test comes and it realizes that it has no way of avoiding being exposed, it will usually reveal itself and either attack the other people around it or try to escape.
However, the test isn't always foolproof: sometimes a very clever impostor will think of a way to either beat the test or make it look like a different person failed the test. In such a case, another strategy, in which the Impostor Forgot One Detail, can come into play.
Compare Spot the Impostor, where an impostor is identified using psychological means such as asking each person for a Trust Password or Something Only They Would Say. Also check out its Sister Trope, If You're So Evil, Eat This Kitten! where it reveals their true allegiance rather than identity.
- A Brazilian Coca-Cola ad is set at a space station where the Captain informs the rest of the crew there's an alien disguised as one of them and, to expose it, he'll ask them a question he expects every Earthling to know the answer. A crew member brings up the fact they came from several countries as a reason to believe not everyone will know the answer but the Captain states that anyone from Earth will know. He asks what the world's best soda is and all Earthlings answer "Coca-Cola". The alien is exposed.
- Near the end of Parasyte, the military figures out that people who've been taken over by the parasites can be identified by looking at x-rays of them — the parasites don't have skulls. They root out the parasites hiding among the people at City Hall by leading them past a large X-ray machine.
- Subverted during YuYu Hakusho. Patches were placed on the protagonists that were originally designed to indicate if the characters were harmed, but doubled as a way to indicate an impostor as only the person who placed them on another could remove them. It didn't work, because the psychic imitator's powers that so exact, so they resort to Spot the Impostor tactics instead.
- In Digimon Ghost Game, Gammamon and several other victims are replaced by a horde of Betsumon. Gammamon unintentionally subjects his imposter to one of these by stress-eating a mountain of chocolate which the Betsumon tries and fails to copy, tipping off Hiro to what was going on.
- Played for Laughs in an episode of Pokémon the Series: Diamond and Pearl. Two Ditto have snuck into the group's camp by impersonating their Pokémon. In an attempt to weed out the impostors, they return some of their Pokémon to their Poké Balls, but when one of the Ditto impersonates Piplup, Ash's Gible uses Draco Meteor, which has a tendency to home in on Piplup.
- In Naruto, an imposter outs himself by remembering the Trust Password Sasuke set up, which Sasuke knew that Naruto wouldn't possibly be able to remember the entire thing, and the imposter had been spying on them.
- Happy Heroes: In episode 27, the gang asks several questions to the two versions of Happy S. to find out which one is the fake one. The fake one wins the test, much to the utter shock of the real Happy S.
- In Pleasant Goat and Big Big Wolf: Mighty Little Defenders episode 27, Weslie finds the other goats trapped in cages but with several of Wolfram's mud golems in each cage posing as each goat. Weslie's initial methods of trying to pinpoint the real ones fail since the golems thought a couple of steps ahead (The golems are ordinarily weak to water, but the goats' doppelgangers applied waterproof lotion to themselves. They also closely studied the actual goats' lives and personalities and can answer Weslie's questions about them easily). In the end, Weslie gets the gang's weapons back from a couple of other golems and uses them to determine who is who, since the weapons can only be used by their respective owners.
- Futurama: Spoofed during the "Time-Bender Trilogy", when Bender winds up in Salem, where the locals have noticed things are going wrong, and conclude there must be... a robot in their midst! They then accuse one of their number, quite plainly not a robot of any kind, and subject him to the trials. A perplexed Bender watches on, informed by Samantha, who definitely is a robot, that the locals were stupid enough to ask robots for a list of their weaknesses, leading to tests such as "robots feel no pain when their hair is cut", "robots are ticklish" and "robots float in water".
Samantha: Luckily, prejudiced people are morons.
- Sgt. Fury and his Howling Commandos: Nick Fury is suspicious that one of the "prisoners" in a camp escape is a German spy. After a long hike, he has Howler Gabe take a long sip from a thermos of water and then offer it to the rest. Sure enough, the Nazi is the one who, despite obvious thirst, refuses to drink from the same thermos a black man just used.
- In The Fake Smurf, Gargamel uses a spell to transform himself into a Smurf to get revenge on them, but there's a small problem - no tail. So he improvises by making a fake one using a wooden fake tail, some blue paint, and glue. Unfortunately for him, when he's caught in his own attempt to sabotage their bridge, the fake tail falls off, a Smurf finds it, and brings it to Papa Smurf. When he realizes this must mean an impostor is around, he pinches the Smurf on the tail to make sure his is real, then the other Smurfs start doing the same to each other, and Gargamel's cover is quickly blown.
- All Assorted Animorphs AUs: In "What if they saved Jake's family?", Eva makes the Berensons drink instant maple and ginger oatmeal when she encounters them on the way to the Hork-Bajir valley. If they can still form coherent sentences in an hour, they're not a Controller.
- In Escape from L.A., Eva makes Tom drink instant maple and ginger oatmeal when they reunite at the church shelter to make sure he hasn't been reinfested since she last saw him, since it makes Yeerks go crazy.
- In How I Live Now, Tom, Jake, and Marco find Rachel out in public, even though she should be dead, and wonder who's impersonating her. You can't acquire someone in morph, so Tom tries to acquire "Rachel" to check if it's really her. It works.
- In the The Familiar of Zero fanfiction The Steep Path Ahead, the Vampires Amethyst and Daphne test the blood of whoever comes to claim to be Karin's daughter. They're giddy when they taste Louise's blood, confirming it's her. Likewise, her manticore familiar can smell whether or not they are the real deal.
- Discussed in Star Wars: Legends Never Die when Luke Skywalker and Mara Jade of Star Wars Legends find themselves in the events of The Force Awakens and Luke has to convince Han that he's real. While certain details of Luke and Han's pasts are different between universes, Han appreciates that Luke mentioning a past wedding where 'his' Han left a woman at the altar because he wasn't ready is too specific a detail about someone he's never met to be completely random, and Luke still has a similar fighting style to his other self.
- In Iron Kissed, the touch of iron is anathema to Fae, causing them pain and breaking their glamours. For this reason, touching iron is occasionally used by those in the know to prove that they are not Fae. Cat Noir, however, is "Iron Kissed", able to touch iron without being harmed, and abuses this to maintain his cover.
- In Ranma's Sudden Wedding, Ranma figures out that the Tendo sisters have been possessed by cursed dolls, and tests this theory by pointing at "Kasumi" and referring to her as "Akane". When "Kasumi" moves towards the currently female Ranma, the dolls' cover is blown.
- In Hawkmoth Gets a Reference, Felix had disguised himself as his cousin Adrien and sent out an insulting message to the latter's friends. Adrien's friends realize that "Adrien" is an imposter because of this behavior, especially since he called Marinette a bitch, and all of them sans Marinette get Akumatized into the monstrous Revealer to hunt him down. When Revealer finds that Adrien and Felix still look like each other, they play back the clip of Felix insulting Marinette, relying on Adrien getting enraged on Marinette's behalf and Felix's shock that Adrien is dating Marinette to figure out who is who.
- Oh, You Don’t Have to Do That. (Communication): After Lila uses an Akuma to disguise herself as Marinette so she can try (and fail) to frame her, Nino and Kim come up with a test to figure out which one is the real one by asking Ms Mendeleiev to create a strange liquid and asking both girls to drink it. The drink turns out to be a concentrated sour flavoring, which knocks out Lila but leaves Marinette unaffected since she can't taste sour.
- Happy Heroes: The Movie: Trying to distinguish the two Doctor Hs, those who happen to be with them pose to them the challenge of fixing a large, complex spaceship. (It was also a convenient way to get that spaceship fixed since they didn't have the funds to do so otherwise.)
- The Thing:
- In The Thing (1982), poking people's blood samples with a hot needle is used to identify the Thing since the Thing's blood will react to try to defend itself when endangered.
- In The Thing (2011), they use a different test that can't tell who is a Thing but can ascertain who isn't: it can't replicate inorganics. Like dental fillings, for instance. If you have cavities...
- In The Faculty, after the students learn that Zeke's homemade drugs are fatal to the aliens, they force everyone in the group to take the drugs to make sure that none of them are spies. Delilah is exposed as being under the aliens' control by the test, and trashes Zeke's drug lab and runs off. Marybeth is also an alien (the Hive Queen, in fact), but manages to beat the test by sealing her nostrils. Everyone else gets really high and starts laughing hysterically. They later try to force the principal (who is infected) to do the same, but she refuses, causing them to shoot her dead and dump the drugs on her body to keep her down.
- The Voight-Kampff test from Blade Runner, used to determine whether the subject is human or Replicant.
- In Screamers, the protagonist cuts a female soldier to ensure she's not one of the increasingly advanced killer robots. It turns out that the latest models can bleed too.
- Averted with this hilarious exchange from X-Men, during a confrontation with Mystique:
- In The Terminator, one reason why Kyle Reese literally pets the dog is because dogs can detect Terminators (by furiously barking their heads off). In the robot-dominated future, members of La Résistance have at least one dog hanging around the entrance of their bases to administer this test on all who enter.
- In Terminator 2: Judgment Day, the Terminator calls John Connor's home to see if the T-1000 was there. Noticing John's dog was barking its head off in the background, the Terminator asks if the dog was okay, referring to the dog by an incorrect name. The T-1000 doesn't realize this was a fluke and exposes itself.
- The World's End: To prove that they haven't been replaced by "blanks", the characters decide to show each other the scars, tattoos, corrective surgeries, or other modifications they got over the years. Gary refuses to do so because they'd see that he'd attempted suicide, instead bashing his head several times against a support beam, showing that he's not Made of Plasticine and doesn't bleed blue ink.
- In the final scene of Police Academy 6: City Under Siege, the heroes are in the same room with Commissioner Hurst and the Big Bad, who is disguised as Hurst. Their test to reveal the imposter is called "the Pinocchio test" and is rather simple. (They perform a nose pull on both commissioners, revealing the imposter to be wearing a rubber mask.)
- Ronin (1998): Sam tests some of the hired mercenaries in subtle ways. But the defining moment was when he exposed Spence, who claims to be part of the British SAS, as a fraud by calling him out not only on his tactics to take out the group's target but asking him a question that shouldn't have flustered a real SAS soldier. Notably, after the fraud has left, someone asks Sam the same question he was asking Spence; Sam shrugs it off with "How the fuck would I know?" Given that all SAS recruits are trained in resisting much harsher interrogation than repeated questioning, it doesn't matter that Sam didn't know the right answer to his own question, only that Spence couldn't keep his cool well enough to even attempt a bluff.
- The Assignment (1997): The plot involves Annibal Ramirez, who bears an uncanny resemblance to the terrorist Carlos the Jackal, being recruited by the CIA to impersonate Carlos. A terrorist who knows the real Carlos is surprised to see him at the duty-free counter at Heathrow Airport because he left Carlos back in Libya.
Terrorist: [quietly] What are you doing here?
Annibal: [winging it] Buying cigarettes. What are you doing here?
Terrorist: [politely] Excuse me, sir, I only wanted to know where I can get a newspaper.
Annibal: Libya, quite a tragedy. [leaning close] I told you never to address me in public. Over by the phone booth and wait; I'll come to you. [goes to leave]
Terrorist: Excuse me, sir. [jams carry bag concealing a gun into Annibal's ribs] I asked... if you knew... where I can get a newspaper. [Oh, Crap! look on Annibal's face] You make the slightest move, and I shoot... and I don't miss.
- Played for Laughs in Ernest Goes to Jail with Evil Twin Felix Nash. He makes zero attempt to look, act, or sound like Ernest aside from his physical looks and wearing most of Ernest's iconic outfit, acts blatantly Out of Character around them and does things they know Ernest would never do, and gets caught red-handed robbing the bank and they still fail to realize it. He gets so fed up with it that he outright tells them he's going to rob the bank and they still fail to realize he's not Ernest. It's not until he applies an impostor-exposing test to himself, after having tied up the bank employees, by pointing out how well and competently he's been doing things that they finally realize he's not Ernest.
Nash: That's it. Is everyone who works here a moron? Can't you see what I'm doing? I'm robbing the bank. I'm gonna blow the safe, take the money, and leave. I'm robbing the bank. I'm stealing the money you are paid to protect. I'm robbing the bank.
[Chuck just starts to laugh]
Charlotte: Look, I don't know what's gotten into you, but you can't go through with this! Ernest! Ernest!
Nash: Look! I am not this Ernest guy. I just happen to look like him. I switched with him, get it? Would the real Ernest be able to knock out Chuck? Or set a time bomb without it going off in his face? And look at the floors. Notice how clean they've been lately?
Charlotte: You are an impostor!
- In Invasion U.S.A. (1952), about a Soviet invasion of the USA, Soviet infiltration units disguise themselves as American soldiers in order to get close to the White House. They claim to be from a Chicago unit, and they speak pretty good English, but they don't know nearly enough about American culture to fool the guards.
Guard: You ever go see the Cubs play?
Infiltrator: [confused] Cubs? A cub is a young animal, a bear...
[Blast Out ensues]
- Animorphs: The Unexpected. Cassie, hiding from the Yeerks on an airplane, tries to pose as a passenger. The Yeerks, knowing she's the only one on the plane who hasn't been affected by their paralysis-inducing phlebotinum, ferret her out by shooting everyone with low-intensity Dracon beams and seeing who flinches.
- Also happens in “The Capture” when the fact that a Yeerk has taken control of Jake is confirmed by watching when Ax comes around him. Yeerks can’t hide their hatred of Andalites.
- The climax of the first BattleTech Expanded Universe novel, "The Sword and the Dagger," hinges on determining which of two apparently perfectly identical, similarly knowledgeable Hanse Davions is the real ruler of the Federated Suns. Due to the intrigues of the rival nation responsible for the duplicity, Hanse's close friends and other allies are discredited in the eyes of the public and their word is not trusted... but there is one thing that cannot be bribed, tricked, or otherwise falsified. (Strangely enough, this is not a DNA test, possibly due to the book being published years before DNA testing was widely understood by the public.) Only the true Hanse Davion would know the secret password to re-activate his personal Humongous Mecha sitting on the palace grounds. The first Hanse Davion is able to get the 'Mech to move stiffly, but claims the weapons are non-operable due to age... only for the second Hanse Davion to climb in and activate all of the 'Mech's systems, discrediting the fraud by making the 'Mech move with precision and grace while firing every single weapon on his BattleMaster.
- Books of the Raksura: One drug is harmless to groundlings but temporarily incapacitating to the shapeshifting Raksura and Fell. Unfortunately, most groundlings don't know the difference between the benign Raksura and the Always Chaotic Evil Fell, so the Raksura protagonist is drugged, betrayed, and nearly killed by his own neighbours at the beginning of the series.
- In the Deryni novels, there's a drug called merasha that causes an immediate and violent reaction in Deryni but has no significant effect on "normal" people. During the persecutions, it was used as a way of uncovering secret Deryni. One application is specifically mentioned in the short story "The Priesting of Arilan": whenever a new priest was ordained, the communion wine at the Ordination Mass was spiked with the drug to make sure no Deryni got into the Church.
- Discworld: In Jingo, werewolf Angua sneaks aboard 71-Hour Ahmed's ship in wolf form by posing as a Klatchistan wolfhound. Ahmed quickly catches her, however, by having the dogs eat from silver plates.
- The Dresden Files: Murphy has Mort cut himself in Ghost Story before inviting him inside. A lot of supernatural beings that require an invitation to enter a building will bleed ectoplasmic goo rather than blood. The invitation, or lack thereof, is another such test in and of itself — an impostor using magic will either not be able to enter, or not maintain their disguise. Murphy pulls this one on Dresden himself, after, in a previous book, being attacked by someone taking his form.
- In The Girl from the Miracles District, Kosma has an enchanted chain that, when put on a berserk, will induce Glamour Failure if they've been taken over by their beast.
- Magic: The Gathering: In the novel Planeswalker, when Xantcha is accused of being a Phyrexian, she cuts herself to show that she bleeds blood rather than Phyrexian oil. However, she actually is Phyrexian: as a sleeper agent, she was created specifically to be able to pass this kind of test. Fortunately, given that as a first try she's actually very poorly equipped to pass for human, she's the first fully organic Phyrexian the locals have encountered and this satisfies them. (Much later, she finds records implying her replacements' generation, despite improvements such as having a physical sex, were all exposed and killed.)
- Partially subverted in the German SF novel Der Mann von Oros, where a blood test is used to dramatically reveal the shapeshifting alien whose frozen 'corpse' was taken aboard the vessel rescuing the surviving members of the stranded Pluto expedition...but fails to similarly detect a second alien who had already replaced one of the castaways on Pluto weeks before and thus had more time to perfect his disguise. This second alien is the 'man from Oros' alluded to in the title and the story's protagonist.
- Played with in Asimov's short story No Refuge Could Save. A spy is tripped up by being familiar with the third verse to "The Star-Spangled Banner", something no true American would knownote .
- In Sunshine, when Sunshine and Con are being interrogated by the police, Con is exposed to sunlight as they suspect him of being a vampire. He is a Friendly Neighborhood Vampire, but Sunshine manages to use her magic to keep him from not frying and hence passing the test.
- "Who Goes There?", the story that inspired The Thing (1982), used the same type of blood tests as the movie to identify the alien.
- Alias: "Project Helix" can make one person exactly like someone else, and there are three main ways to uncover the imposter. Of course, the imposters know all about this and spend lots of time framing innocent people.
- Provacillium, a medication taken by the imposters.
- An eye test that analyzes proteins in the retina.
- Out-of-Character Alert. "I just remembered: Francie doesn't like coffee ice cream."
- Gaius Baltar spends most of the first season developing a Cylon detection test in Battlestar Galactica (2003). Unfortunately, his self-serving, cowardly, and at times downright stupid nature mean that even when test does expose someone he doesn't tell anyone the truth. After encountering a real Cylon that the rest of the fleet thinks passed the test, they assume the whole test was flawed. Interestingly, the first time he "invents" the test, he uses it to try to direct the people's attention to a device he doesn't recognize, so he picks a random guy he doesn't know and "reveals" him to be a Cylon. The guy is abandoned on a supply space station... and then he's revealed to really be a Cylon.
- Doctor Who: In "Smith and Jones", the Judoon have scanners that can distinguish humans from non-humans, which they try to use to find a plasmavore criminal hiding in a hospital. The plasmavore is able to change its physiology by drinking blood, and tries to use that to beat the test; however, the Doctor tricks it into feeding on him. Since he's a Time Lord, the scanners identify the plasmavore as non-human and kill it.
- On Father Dowling Mysteries, Frank has to handle the antics of his twin brother, Blaine, a con artist who often poses as his priest brother for scams. The two are dressed alike when the cops try to arrest them both. Sister Steve asks Frank what a patron told him in confession and he says he can't repeat it. Steve thus knows it's him as the real Frank would never break the sanctity of confession even to save himself.
- In First Wave, the Gua/human hybrid bodies the Gua use have built-in mechanisms that dissolve the body moments after death in order to hide evidence of alien presence. They also rapidly heal from wounds. When Cade wakes up after an explosion, he is told that the government now knows the truth and is starting a manhunt for the Gua. Cade begins to suspect something and, eventually, holds one of the agents hostage. In order to prove he's human, the agent sticks out his hand, and Cade stabs it. The wound doesn't heal. Later on, it's revealed it was a Gua operation, and the hand was deliberately engineered not to heal.
- Fringe: The sinister cyborg shapeshifters have mercury for blood. Blood screening is standard procedure when shapeshifters are at large.
- Haven: Played straight and subverted in two separate instances in the episode "As You Were," when a shapeshifter decides to do a Kill and Replace at Audrey's birthday party. Subverted when the group decides to suss out the imposter by having everyone reveal what gift they brought, since none of them are open yet. Nathan fails this test and is restrained, but he isn't the imposter: his girlfriend rejected his gift idea and replaced it without telling him. Played straight when Audrey unties him after this information is revealed. Nathan can't feel her touch, though with Audrey's Anti-Magic against the Troubles, he should be able to. He asks if she trusts him, and then kisses her. He can't feel it, meaning the Imposter Forgot One Detail and this Audrey is a fake.
- Journey to the West (1996) has a chapter where an absurdly powerful Centipede Demon takes on the form of Sanzang, resulting in having two identical Sanzangs much to the confusion of his disciples, Wukong, Bajie and Wujing. After Wukong's X-Ray visions failed to expose the fake Sanzang from the real one (a trick which worked 99 out of 100 times in the past - the Centipede Demon is just that good of a shapeshifter) the trio decides to quiz the two Sanzangs on their past adventures, leading to a nice Continuity Cavalcade that Call Backs on the previous chapters, with questions like, "Who is my (Wukong) brother-in-arms in my training days?" note "Where did we first met when I (Bajie) pretended to be a rich nobleman?" note "What did you write in the palm of my (Wujing) hands in the bottom of the River of Sands?" note "Who defeated me (Wukong) in battle and reverted me back to monkey form?" note "In which country did I (Bajie) become a prince?" note "What is my (Wujing) occupation in my previous life, a hundred years ago, before my exile from heaven?" note … to the horror of Wukong, Bajie and Wujing, BOTH Sanzangs answered all twenty or so of their questions correctly, making them realize the Centipede Demon is likely a mind reader and far more powerful enemy than they've previously faced.
- K.C. Undercover has KC and her clone (in different clothing) undergo a series of questions about her personal life. The impostor gets the first two questions right, then answers the third (on a really embarrassing childhood incident) in detail without missing a beat. She didn't call it "my number one mistake."
- Lucifer (2016): In season 5A, Lucifer's Evil Twin Michael arrives to do a Twin Switch while his brother is ruling over hell, basically to mess things up and ruin his life. This includes Lucifer's relationship with Chloe Decker. Chloe, however, isn't a detective for nothing and figures it out pretty quickly that Lucifer is Not Himself. She manages to nearly convince Michael that they're on the verge of a break-up, before pulling her gun and shooting him in the leg. Chloe knows that Lucifer, despite being an immortal celestial being, will bleed if she shoots him, because his feelings for her make him vulnerable. Michael pulls his hand away from the wound to reveal no blood, and the jig is up.
Chloe: So, who are you?Michael: I'm Michael, Lucifer's brother. Nice to meet— (Chloe shoots him again) You know that won't kill me. (another gunshot)Chloe: Yeah, but it makes me feel better. (shoots him one more time for the hell of it)
- In the Red Dwarf episode "Psirens", a mind-reading shapeshifter takes on Lister's form. The crew manage to discover who is who by asking them to play the guitar. The Psiren reads Lister's mind, unwittingly picking up on Lister's delusion that he is a virtuoso, rather than a lousy player, and plays brilliantly. The Cat and Kryten then shoot him.
- Subverted in Sanctuary. Magnus, Will, and a few one-shot characters are trapped in a crashed plane in the Hindu Kush mountains with a shapeshifting abnormal. Realizing that the creature lives in incredibly cold climates, they decide to draw some blood from everyone and freeze it; the blood that doesn't freeze belongs to the imposter. They don't realize that the abnormal can make them see anything it wants, so the test is pointless. The real test is more spontaneous on Magnus's part, when she asks "Will" to get her some coffee, having earlier told Will how much she hates the stuff. "Will" is all too happy to oblige, revealing himself to be a fake.
- In the Stargate SG-1 episode "Foothold", Maybourne cuts himself in front of Carter to prove he's human after an alien impostor is shown to have purple blood.
- Rather thoroughly deconstructed in Star Trek: Deep Space Nine. The shapeshifting changelings caused paranoia about their infiltration abilities. As such, Starfleet briefly initiated required blood tests of officers and their family, as any blood removed from a shapeshifter's body would instantly revert to protoplasm. It works the first time because it's fairly spur-of-the-moment, but once it's established as standard policy, it becomes less effective, since the changelings now know about it and can work on a way to defeat it. There's one scene where a character initiates one of these; a season later, it's revealed that he was a changeling the entire time.
- Also the Day of the Jackboot conspirators fake the test to frame Sisko as a shapeshifter once he finds out about their plan.
- Played straight the first time it was used ("The Adversary") in an obvious Shout-Out to The Thing; the shapeshifter escapes the moment it's asked to take the test; presumably a means of passing the test had not been worked out yet—or possibly they did it deliberately to convince those present, if they should survive, that the test works.
- Star Trek: Discovery:
- A mild version: having already read up on him, Lorca quizzes Ash Tyler, recently freed from a Klingon prison, about his history in the guise of polite interest and small talk. Lorca points out a minor inconsistency (he grew up outside Seattle, but just said he was from the city when asked). Lorca lets it pass, as it's an obvious shorthand to use, and he got everything else right. Ash also quickly figures out what's going on and both acknowledge that it's what they were doing.
- In the season 1 finale, Burnham tries to ask "Captain" Phillipa Georgiou a series of questions about her background in order to expose her as ex-Emperor Phillipa Georgiou of the Terran Empire. However, Burnham fails, as Mirror Georgiou has studied up on her dead Prime counterpart.
- Star Trek: The Original Series:
- In "The Trouble with Tribbles", the Tribble dislike for Klingons is used to identify the Klingon spy disguised as a human. It's one of the reasons the Klingons embark on a "glorious" campaign to slaughter every Tribble in existence (although the fact that they utterly destroyed the biosphere of any planet they landed on was slightly more pressing). They succeed. Then, in the Star Trek: Deep Space Nine episode "Trials and Tribble-ations", someone brings a few Tribbles from the Kirk era, and given that they're Explosive Breeders...
- In "The Paradise Syndrome", Kirk is knocked out and afflicted with Laser-Guided Amnesia while visiting a planet inhabited by Native Americans, transplanted there by Ancient Astronauts. When they find him at the "holy" Precursor monolith, everyone (including Kirk himself) thinks he's the god "Kirok"... until one of them cuts him, and contemptuously exclaims, "A god who bleeds!"
- Season 2 of Supergirl (2015) shows that Martians, both White and Green, turn to their real form when close to fire, which the heroes use to weed out the infiltrators.
- When Sam comes Back from the Dead he ties Dean up so that he won't attack him, then cuts himself with a silver knife and swigs a mouthful of salty water to prove he's really himself.
- When Dean returns from the dead a few seasons later, Dean puts himself through a battery of tests, including cutting himself with a silver knife and Bobby throws holy water in his face. Sam and Bobby remain suspicious of Dean until they figure out what brought him back.
- In another episode, a parasite has infected one of the characters, but they can't be sure who. They had earlier figured out that electricity was so effective on the parasite that it would be forced to leave the host, so the characters had to take turns shocking themselves to prove they didn't have it.
- When Sam and Dean meet their long-lost half-brother Adam, Dean puts him through a battery of tests, all of which he passes because it's actually a ghoul who killed and ate Adam and there's no test for that.
- In the seventh season, it's shown that leviathans have Black Blood. This trope is implemented when Frank and Dean have a gunpoint confrontation in which Dean cuts his arm to show Frank his blood (though he had to talk Frank out of shotgunning off Dean's foot instead). Frank does the same at Dean's insistence.
- In general, Supernatural has a lot of these, including drinking salt water (anti-ghost), touching holy water (anti-demon), cutting yourself with silver (anti-shifter), and touching borax or showing that you bleed red (anti-Leviathan.)
- Of course you need to be looking for the right thing in the first place. When a demonic virus infects a town, the protagonists tie up a potential infected for several hours until his bloodwork clears him. After he's released, it turns out he was under demonic possession instead.
- An interesting variation in "Celebrating the Life of Asa Fox". A demon is Body Surfing between various people, so everyone present knows someone is the demon but they aren't sure who. Unable to access any holy water or salt lines, Dean begins drawing a Devil's Trap. As Dean points out, a non-possessed person can step into the trap, then step right out, and will have no possible reason not to do so, the demon alone will refuse. Realizing the jig is up anyway, the demon reveals itself and attacks.
- And finally, a failed version. In the Season 1 finale, Sam and Dean rescue their father who had been captured by the Yellow-Eyed Demon. To make sure he's not possessed, they throw holy water on him before grabbing him and running. It later turns out he is possessed by Yellow-Eyes himself, who is so powerful that he's actually immune to holy water. Instead, he gives himself away by being out of character.
- The Wheel of Time (2021): In episode 8 Ba'alzamon places Rand into a scene of a perfect future, where he lives with Egwene and their daughter in Two Rivers. Subversion: Rand asks about things only the real Egwene knows, but she gives answers that match his memories of her. Double subversion: this perfect Egwene did not leave him to become Wisdom or Aes Sedai, thus he deduces she's fake or brainwashed — not the woman he wants to be with.
- In Iron Kingdoms's game Warmachine, if more than one of the same character appears in a game then one of them is an impostor, with the one who's on the winning side being the real one.
- Eberron: Since changelings are a known quantity, various non-magical imposter tests are common practice even among normal people. From adopting an Iconic Outfit to a simple Trust Password or even a Secret Handshake, people make an effort to test double-check their friends as a matter of course. However, these methods are largely useless because changelings only rarely bother impersonating actual people. More commonly, they'll make up a whole new persona that might then be shared around the community. So your friend Hank the Fighter knows all your paranoid passwords because he's been a changeling—or even multiple changelings—this entire time.
- Among Us has this when task animations are turned on. Certain tasks will trigger an animation when completed, visible to all players. Most notably, the Medbay Scan is an actual test to prove your identity as a crewmate; other crewmates viewing you complete the task will know you are the real deal. Impostors can't complete this task, so a rookie impostor claiming to have that task but refusing to do so is considered extremely suspicious. Conversely, anyone seen scanning is proven to be a crewmate.
- The Voight-Kampff test in Blade Runner. It functions as an important tool for identifying replicants, as well as a unique and engaging game mechanic. The test automatically terminates after asking ten questions, regardless of whether a conclusive result has been obtained. The player can choose the nature of the questions, ranging from simple calibration questions (like those of a polygraph test) to enormously provocative questions involving child abuse and animal cruelty. In some cases, the questions the player chooses can be the difference between the test identifying the subject as a human or as a replicant.
- Spy checks in Team Fortress 2. Thanks to Friendly Fireproof, you can shoot your own teammates without hurting them, but if you shoot a Spy disguised as one of your teammates, they'll take damage. The Pyro's flamethrower is the classic choice since only Spies will catch on fire. However, the spy has weapons in his arsenal which can enable short immunity to fire (like the Spy-Cicle knife) or fake a death (like the Dead Ringer watch).
- The princess in Shining Wisdom has been replaced by a demon; however, the real princess has a tiara that renders her impervious to damage, so the only way to figure out who the real one is is to attack her. The King is rather reluctant to do so.
- Numerous maps in Trouble in Terrorist Town will include a "traitor detector"; a small chamber that will determine if its occupant is a traitor or not based on a panel of rednote and greennote lights. To make it fairer to the traitors, many maps that have one also have some caveat to using it, such as making the players complete a secondary action to make the detector work, requiring multiple people to be in the machine at a time in a Twelve Coins Puzzle scenario, or putting a "traitor-only room" near the detector, giving the traitor some incentive to go near it.
- Used as a mechanic in The Thing (2002). Blood test kits consist of a syringe pistol that draws a blood sample, and then adds a chemical that reacts to it by producing heat. If the blood cooks and turns brown, they're a person. If the syringe breaks, they're a monster. However, the test isn't foolproof for story reasons.note
- At a point in The Witcher 3: Wild Hunt, Geralt is offered a drink. When he accepts, he's told that he just passed such a test, as the silver cup would have burned the lips of a doppler who was assuming Geralt's identity. Geralt can point out that he's carrying a silver sword, but the other party will claim that dopplers can turn their body into materials that look like silver but aren't.
- In Fallout 4, the player can encounter a small town called Covenant that has a test designed to root out Synthsnote . The question that supposedly trips Synths up is "What position would you want to play on a baseball team?"; Synths will (apparently) instinctively answer "catcher" even if they don't know what a catcher actually does. However, the test's efficacy is questionable at best, which is used to show how paranoid the people of Covenant are since they're willing to treat a single multiple-choice question as damning evidence. The only guaranteed way to tell if someone is a synth or not is, unfortunately, to kill them and check their body for a Synth Component (which doesn't appear in their inventory while they're alive as it's presumably inside of them).
- The Jackbox Party Pack: "Push The Button" from Party Pack 6 involves players taking a series of tests, which range from drawing a picture to answering a question (multiple choice, agree/disagree, or fill-in-the-blank) to determine which of them are aliens and which are human. Aliens are given prompts to answer that are different from other players (with prompts later in the game often becoming wildly different), but they also get a limited number of "hacks" they can use to give a correct prompt to another alien player or an incorrect prompt to a human player.
- In Yakuza: Like a Dragon, Ichiban ends up doing this when Adachi gets into a scuffle with an assassin disguised as him. He does this by quizzing them on traffic laws since Adachi's former day job was a DMV instructor, with one answering perfectly and the other answering in complete bafflement. He proceeds to punch out the one who answers correctly (to the bafflement of Saeko), who then declares the real Adachi to be a stain on civil servants.
- In Final Fantasy XIV, Themis and the Warrior of Light encounter Hephaistos, who is claiming to be Lahabrea. When Themis asks him to recite his name, he answers with Themis, which confirms the boy's suspicion that Hephaistos is not who he claims he is. Themis later states that the real person would have answered with Elidibus, which is his true name.
- One episode of Martin Mystery, as a Whole-Plot Reference to The Thing (1982), naturally had this. In this case, the test consisted on scanning the DNA of hair and saliva.
- Transformers: Animated has this as Wasp impersonates Bumblebee. Their chosen contest? The video game Bumblebee had been playing at the beginning of the episode.
- In one episode of The Smurfs, Hogatha uses a spell to transform herself into a smurf to get revenge on them, but there's a small problem - no tail. So she improvises by making a fake one using a pea, some blue paint, and glue. Unfortunately for her, when she's caught in her own attempt to sabotage their bridge, the fake tail falls off, Clumsy Smurf finds, it, and brings it to Papa Smurf. When he realizes this must mean an imposter is around, he pinches Clumsy on the tail to make sure his is real; then the other smurfs start doing the same to each other, and Hogatha's cover is quickly blown.
- In one 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 Dojo has pulled the carpet over my eyes!Clay: Yup, that's Omi all right.
- One Looney Tunes short had Henry Hawk try to determine whether Foghorn, the Barnyard Dawg, or Sylvester is a chicken by lining them up and waiting for morning, reasoning that roosters always crow at dawn. Foghorn uses ventriloquism to make it look like Sylvester is crowing.
- Fraidy Cat has this happen in the final episode. In “A Small Star is Born”, Lawrence the lop-sided lion catches both Boris and Fraidy and has Cling-Clong test and see which one is real and fake. Both Boris and Fraidy are given food, and Boris, being the picky eater he is, puts his nose up to the food. Fraidy, however gorges upon the dish, and he is literally kicked back to the city.
- On Regular Show a shapeshifting otter tries to take Rigby's place and the others can't tell them apart. To try to figure out which one's the real Rigby, they go through a series of tests, all of which they "pass" by failing miserably. It's finally when they ask what Rigby's secret fear is that the one that answers is declared the real Rigby and is given a hug by Mordecai... which outs him as the imposter because the real Rigby would never hug Mordecai.
- In one episode of The Flintstones, Fred falls asleep at the company picnic and dreams he pulls a Rip Van Winkle, sleeping for decades. Unable to find Wilma when he wakes up, he eventually finds Barney, who's now a millionaire. Barney isn't sure it's actually Fred, however, seeing as he's been missing for years and fakes keep showing up trying to get their hands on Barney's money, but he knows how to make sure: he calls for Dino. When Dino sees his old master and responds the same way he always did (which is happily leaping and tackling Fred, then licking his face) Barney knows it's really Fred.
- The Transformers:
- The Autobots are forced to come up with a series of them in "A Prime Problem," where Megatron creates a clone of Optimus and uses it to sow confusion in the Autobot ranks, especially when the real Optimus comes back and both insist they're real. The computer scans indicate they are physically identical... which means the Autobots are holding a collective Idiot Ball when they have the two Optimii engage in physical tests to determine the real one. They are finally tipped off by the fake Optimus acting with uncharacteristic dismissal of humans in danger.
- In "Masquerade," the Autobots capture and impersonate the Stunticons to try and foil a Decepticon plot. The real Stunticons escape and confront their impostors in front of the other Decepticons. Megatron calls for them to unite into Menasor, thinking the fakes won't have the ability to do so. The Autobots-in-disguise pull a fast one by employing the holograms of Mirage and the magnetic fields of Windcharger to look like they're forming their own Menasor. As before, the test finally falls apart when they start fighting and the actual violent psychopath knocks out the Autobots.
- In The Scooby-Doo and Scrappy-Doo Show, the main characters have to deal with a Master of Disguise who, unlike the other people in disguise over the franchise, chooses to impersonate the main characters. They quickly learn that he cannot stand Worcestershire sauce and will gag, choke, and loudly complain in his natural voice if fed even the slightest amount of it. The rest of the episode is this trope Played for Laughs as he is repeatedly unknowingly fed food with Worcestershire sauce in it, giving himself away in front of everyone each time.
- Used by Jem herself when Clash is faced with her in "One Jem Too Many." Bonus: Samantha Newark, who voices Jem, is doing Clash's vocals here.
- This trope serves as the climax of the My Little Pony: Friendship is Magic episode "Too Many Pinkie Pies." Pinkie, having a hard time choosing which of her friends to play with, decides to use a magic pool to make copies of herself; the other Pinkie Pies will visit the mares Pinkie doesn't see and report back to the original about the activities. Unfortunately, the clones only take on Pinkie's fun-loving, goofy side and cause chaos wherever they go, as they lack any empathy for the ponies around them. When the rest of the Mane Six can't find the actual Pinkie Pie in the group, they gather all of the copies in the town hall and reveal a test...quite literally watching paint dry. Twilight correctly reasons that all of the clones will become too bored and try to have fun rather than stay perfectly still and watch—only the real Pinkie would be willing to do such a horribly bland activity if it meant getting to stay with her friends. The test works and the true Pinkie rejoins the group, learning a lesson on having to make choices about how to spend time in the process.
- Punky Brewster: In the episode "Double Your Punky", Glomer creates a clone of Punky from a photograph to keep him company at home while she's at a school picnic. This Punky clone is obnoxious, however, and in his confusion to corral her, he zaps the sweet Punky back into the photograph. When the two Punkys face each other, nobody can tell who is who until her dog Brandon, who recognizes her scent, jumps on her and licks her face profusely.
- During Ganon's attempt to replace Zelda with an Evil Twin in The Legend of Zelda (1989), Link deduces early on that he's with an imposter Zelda due to her lacking any reflection when they pass by a body of water and plays along with her as he finds a way to tell the real one apart when they confront her. Upon learning that the imposter is willing to make the moves on him, he eventually decides that best way to tell apart the two Zeldas is through a kissing contest, as he knows the real one would reject to giving Link a kiss.
- In Chip 'n Dale: Rescue Rangers episode "Dale Beside Himself", Dale comes across a shapeshifting alien named DTZ, who upon a brief conversation, takes on his shape and form before eventually deciding to replace Dale entirely. Dale tries to confront him for this, but it is interrupted by Brik and Brack, DTZ's fellow aliens who insist he come back with them, lest the atomize Earth. Fortunately, Dale remembers something the aliens stated earlier during his time on the mothership and returns with a plate of Erkburgles. Upon seeing them, DTZ is unable to control himself, dropping his disguise and revealing himself.
- In Garfield and Friends episode "Binky Goes Bad", when Binky is on trial due to a criminal disguised as him, Garfield arranges for the imposter to end up in the court room. When nobody can tell the clowns apart, Garfield has the judge say "Order in the court", to which Binky says "I'll have a ham on rye, hold the mayo!", proving he's the real one and leading to the imposter being arrested.
Garfield: (To the audience) The real Binky could never resist an old joke.
- In real life this is known as a Shibboleth, after a bit in the Book of Judges where the Gileadites killed fleeing Epraimite refugees by pulling them aside and asking them to pronounce "Shibboleth"note Nowadays, it's become another name for jargon used among an in-group.
- In his account of the making of a Navy SEAL, Damn Few, Rorke Denver recounts the night his SEAL unit went for a beer to find a guy sitting at the bar who was claiming to be a SEAL. His dress, attitude, demeanor, and presentation were subtly wrong and a long-suffering waitress tipped them off that "Billy" used his Seal status to scare people. The least threatening real Seal was sent to quiz the suspicious Billy about where he'd been, who he'd trained with, and what his combat specialties were. He failed on every test. When Billy went to the men's room, the largest and hardest SEAL followed him in. A little discussion ensued and Billy ended up running for his life, stripped of his fake badges—which later ended up pinned to the real Seals' mess-room wall with a combat knife.
- Most Special Forces and elite units have a pretty direct way with posers. The British Parachute Regiment can be extremely direct with fakers. When the film Maroon Beret was filmed in Belarus, the real Maroon Berets (In Russia, the Spetsnaz elites) have actually assembled to approve the actor's right to wear that beret. Having passed through Hell to get there, they do not consider imitation to be a welcome form of flattery and treat this as taking the piss in a big way.
- In a curious counterexample, in the US Special Forces, people of color are generally not present in the same ratios as elsewhere in the armed forces. In at least one case, during a counter-infiltration drill aboard a Navy vessel, the Navy crew (knowing this factoid) won without even having to try just by arranging the duty roster so there was always at least one non-white member in each group of crewmen.