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Impostor-Exposing Test

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Impostor-Exposing Test (trope)
And yet, the canned meat merchant still gets through.
"You see, when a man bleeds, it's just tissue, but blood from one of you Things won't obey when it's attacked. It'll try and survive... crawl away from a hot needle, say."
R.J. MacReady, The Thing (1982)

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? Well, 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.

If the impostor is an alien, they can cut themselves and see who has Alien Blood—or, if they're a robot, see if they bleed at all. If it's a vampire, they can dip their hands in holy water and see who gets burned. More mundane impostors might need to be tripped up by asking for a particular bit of knowledge or hearing how they pronounce a specific word.

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.

This trope is in play when there's a test that will work on any impostor of this type, regardless of who they're impersonating. Also check out its Sister Trope, If You're So Evil, Eat This Kitten! where it reveals their true allegiance rather than identity.

Cover Identity Anomaly is when an impostor is using an invented identity, not attempting to impersonate a specific person, but they get some crucial detail wrong. Characters who suspect that someone has been replaced might be forced to play 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. Compare also Bluff the Impostor, where a character offers up a specific incorrect detail about the suspect's life or memory, to see if the suspect notices it, as a non-obvious test. Impostor Forgot One Detail is when the impostor slips up without being prompted, exposing them as a fake.


Examples:

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    Advertising 
  • 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.

    Anime and Manga 
  • One Piece: During the Alabasta Arc, the Straw Hats wear bandages around their left arms as an identifying mark because they know they'll be up against the shapeshifting Mr. 2 Bon Clay. Bon Clay catches on to the bandages, but fails to recognize there was a second layer to the plan: Under the bandages was an X mark drawn onto their arms, which the Straw Hats would show off to identify themselves as the real deal.
  • 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.

    Asian Animation 
  • 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.

    Comic Books 
  • 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.
  • In Diabolik his plastic masks got exposed fairly early, so the police picked up the habit of pinching people's face to check if it's him every time they suspect his interest. Diabolik being Diabolik, from time to time he still finds ways to get through the checks, such as replacing a cop whose lower face is identical to his and wearing glasses to make sure they won't pinch where there's the mask or just use theatrical make-up if he's not replacing anyone in particular.

    Fan Works 
  • 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.
  • Eleutherophobia:
    • 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.
  • 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.
  • In The Vampire Spawn with the Green Hand and a Tadpole, Astarion winds up as the Inquisitor, and tries to hide that he's a vampire. Solas exposes him by offering him a wineglass of diluted animal blood, and takes his enjoyment (and lack of vomiting) as proof.
  • 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.
  • Starscream (The Movie) [AKA WWIIformers] : Otto, who is German, needs to speak to someone in England, pretending to be American because their countries are at war. Mr. Griffith asks him if he bought his father's book in Washington, if he thought it was too political, and requests that he put the date on a letter. When Otto complies, Griffith pegs him as an German because the book was never published in America due to its inflammatory political content, and points out that he wrote the date in day-month-year order, rather than month-day-year like an American.

    Film — Animated 
  • 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.)

    Film — Live-Action 
  • 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 inorganic substances, such as dental fillings. If you had cavities, they know you're safe, because they can see the metal fillings in your teeth.
  • 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. It works by measuring subconscious body signals—blink rate, skin temperature, retinal dilation—to see if the person being asked deliberately provocative questions has normal emotional responses.
  • 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 1, during a confrontation with Mystique:
    Wolverine: Hey, hey! It's me.
    Cyclops: [ready to shoot] Prove it.
    Wolverine: You're a dick.
    [Beat]
    Cyclops: Okay, it's him.
  • Sinners (2025): While the survivors are holed up inside as refuge from the vampire horde, they decide to ingest cloves of garlic to determine if any of them are vampires. Turns out, all of them were human.
  • 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 RĂ©sistance have at least one dog hanging around the entrance of their bases to administer this test on all who enter.
  • 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.)

    Gamebooks 
  • The Fighting Fantasy book, Crypt of the Sorcerer (an entry that already runs on Nintendo Hard), does this near the end when you're confronted by Razaak's keeper, Ungoth the Skeleton King. Who makes you prove your allegiance with a ton of quizzes, ranging from the mundane ("What's the current world record for the amount of hobbit ears eaten by trolls?") to serious ("How old is Razaak's father when he died?") Fail to answer any riddle or attempt to attack Ungoth, you'll be Taken for Granite on the spot.

    Literature 
  • Animorphs:
    • Because Yeerks are in the brains of their hosts and can access their memories, they can perfectly impersonate those hosts and rarely give themselves away. In “Animorphs: The Capture”, the Animorphs are suspicious that Jake could have gotten infested when a mission went sour. This is confirmed when he sees Ax and the Yeerk in his head can't hide its visceral hatred, flashing it on Jake's face for a split second. The Yeerk tries to play it off and brush off any suspicion, but Jake's friends point out that Jake, while he wouldn't be happy about it, would readily cooperate to show he was uninfested. When Ax touches him and the Yeerk calls him "Andalite filth", the jig is up entirely.
      • Generally speaking, the absolute fastest way to find out if a human is a Controller is for Ax to show up in his normal form. Most can't resist yelling "Andalite!" and trying to kill him. It is not foolproof though: in Animorphs: The Absolute the governor's infested husband, unlike one of the nearby bodyguards, plays it cool. Marco, seeing how cool he played it, suspects anyway and it's confirmed when the husband leaves the room, collects some other infested personnel, and tries to kill the Governor.
    • In Animorphs: The Threat, the Animorphs turn into seagulls to scope out a meeting. Unfortunately, the Yeerks are one step ahead of them: a Controller guards is wearing special sunglasses that fire very low-power Dracon beams, and is watching the seabirds. The real seagulls get hit with the beam, flinch from the pain, and fly away from the building, but don't understand what's going on, so ones that haven't been hit aren't taking evasive action. To keep from attracting attention the Animorphs have to all act like them, getting hit and then wheeling away.
    • 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, try to ferret her out by shooting the passengers one by one with low-intensity Dracon beams and seeing who flinches.
  • The Bartimaeus Trilogy: In Ptolemy's Gate, a meeting of subversive commoners has everyone drink from a silver cup to detect spirits disguised as humans, since silver is deadly to spirits.
  • 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.
  • The short stories in I, Robot contain a couple examples:
    • In Evidence, a suspiciously perfect mayoral candidate, Stephen Byerley, is suspected of being a robot. Various attempts at detection are frustrated in non-suspicious ways—for instance, they attempt to use an x-ray machine on Byerley, but he's wearing a personal shield that blocks x-rays, as an extension of his right to privacy. Eventually Byerley is forced to prove once and for all that he's human by disobeying the Three Laws of Robotics: a random member of the crowd at one of his events comes up on stage and demands that Byerley punch him. Byerley reluctantly does, thus proving he's human. In the epilogue, Dr. Calvin points out the obvious way to defeat this test—if the "volunteer" was a plant who was also a robot—but the story doesn't provide an answer either way.
  • 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.

    Live-Action TV 
  • 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.
    • Some form of Spot the Impostor, such as an 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.
  • 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.
  • Subverted in Sanctuary (2007). 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.
  • Silverpoint: Bea, Faye, Isabel and Charlotte stand watching each other for 16 minutes, as the Alien's disguise periodically refreshes in that interval.
  • 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 (for instance, by simply keeping a small amount of blood in their system that they can draw out as needed). 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.
  • 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...
  • 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.
  • Supernatural: 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.)
    • 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.
    • 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.

    Tabletop Games 
  • 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.

    Video Games 
  • 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.
  • Baldur's Gate III: When you first meet Karlach, she is being pursued by a group of servants of Zariel masquerading as paladins of Tyr. If you're a cleric of Tyr, you can expose them instantly:
    Player: You say you serve Tyr. Recite the Creed of the Left Hand.
    Anders: Wh — I — please, I am wounded. Overcome. I can hardly remember my own name.
  • The Voight-Kampff test in Blade Runner (1997). 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. Just make sure you have it calibrated correctly, or the results will be worthless.
  • 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).
  • 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.
  • 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.
  • No, I'm not a Human: The standard procedure for exposing Visitors is checking for traits such as perfectly white teeth, dirty and damaged hands, bloodshot eyes, et cetera. However, they are not foolproof, as a human may show signs consistent with Visitors... or a Visitor may show signs consistent with humans.
  • 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.
  • 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).
  • 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 
  • 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 12 Coins Puzzle scenario, or putting a "traitor-only room" near the detector, giving the traitor some incentive to go near it.
  • 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.

    Webcomics 

    Web Videos 
  • In the Microfaun short ''The Voided", the hunter Ezekiel meets a guitar-playing man named Judah. Judah mentions that the Voided are unable to lie, and Ezekiel mentions a theory about the Voided being made from the malignant mists forcing out and replacing a person's soul. Ezekiel then theorizes that without a soul, a Voided wouldn't have any sense of identity or self, and therefore cannot refer to themselves (by using words like "I", "me", "myself", or their own name) without lying. Throughout the conversation, Judah had never once referred to themselves at all...
  • At the end of Phelous' review of the Wabuu movie, Phelous is confronted by both the East-West and Dingo Pictures versions of Wabuu, prompting a classic Spot the Imposter situation. However, before Phelous can decide for himself, Dingo Wabuu kills East-West Wabuu with a shotgun. When Phelous questions this action, Wabuu argues that his willingness to murder his doppleganger is, in of itself, proof he's the real Wabuu.
    Phelous: Oh. But how do I know that YOU weren't the evil one?
    Wabuu: Of course I'm the evil one! I just shot that racoon in cold blood!

    Western Animation 
  • In the 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.
  • One Foghorn Leghorn short has Henery 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.
  • Parodied in the Futurama episode "Fear of a Bot Planet". In order to rescue Bender from Chapek-9, a planet inhabited by robots where humans are killed on sight, Fry and Leela are forced to dress up as robots and try and trick their way in. The city guards are suspicious and attempt this, but the test is laughably easy:
    Guard 1: Which of the following would you most prefer: a) a puppy; b) a pretty flower from your sweetie; or c) a large, properly-formatted data file?
    Guard 2: Choose!
    [Fry and Leela confer briefly]
    Fry: Is the puppy mechanical in any way?
    Guard 1: No. It is the bad kind of puppy.
    Leela: Then we'll go with that data file.
    Guard 1: Correct.
    Guard 2: The flower would also have been acceptable.
  • One episode of Martin Mystery, as a Whole-Plot Reference to The Thing (1982), naturally has this. In this case, the test consists of scanning the DNA of hair and saliva.
  • 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.
  • 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.
  • In one episode of The Smurfs (1981), 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 the Transformers: Animated episode "Where is thy Sting?", Wasp pulls a Capture and Replicate on Bumblebee, swapping their appearances down to their voices. To tell who's who, Bulkhead has the two sit down and play a video game together, citing Bumblebee as the best at the game. Wasp, knowing he's terrible at the game and will be exposed, makes a break for it.

    Real Life 
  • 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.
  • During the Battle of the Bulge in World War II the actor David Niven, at that time a British Army officer, was stopped and questioned by suspicious American soldiers who demanded that he tell them who won the World Series in 1943. He replied that he no idea about that but that he had starred in a film with Ginger Rogers.

 
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Alternative Title(s): Imposter Exposing Test

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Catching Copycat

In order to catch the shapeshifting Copycat, the Ghostbusters trick the metamorph into transforming into Slimer, and then, after double-checking which was the real Slimer, trapped it.

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Main / TrickingTheShapeshifter

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