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Impostor Exposing Test
The cast has been infiltrated by a shapeshifter, replicant, robot, or some other creature that is able to pass as human; or else someone formerly trustworthy has lost their humanity to The Virus 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.

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.

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.

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.


Examples:

Anime and Manga
  • 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.

Film
  • The Thing (1982): Poking peoples' blood samples with a hot needle is used to identify the Thing, since the the Thing's blood will react to try to defend itself when endangered. In the prequel, they use a different test that can't tell who is a Thing, but can ascertain who isn't:it can't replicate inorganics, so if you have dental fillings you're human.
  • 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 none of them are spies. One of them is exposed as alien by the test; another is an alien but manages to beat the test.
  • The Voight-Kampff test from Blade Runner, used to determine whether the subject is human or Replicant.
  • In the first Screamers movie, the protagonist cuts a female soldier to ensure she's not one of the increasingly advanced killer robots. Turns out the latest models can bleed too.
  • Averted in this hilarious exchange from X-Men:
    Wolverine: Hey, hey! It's me.
    Cyclops: Prove it.
    Wolverine: You're a dick.
    Cyclops: Okay.
  • In the The Terminator, one reason Kyle Reese literally pets the dog is because dogs can detect Terminators (by furiously barking their heads off); in the robot-dominated future, resistance members 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 he's attempted suicide, but instead bashes his head several times against a support beam, showing he's not Made of Plasticine and doesn't bleed blue ink.
  • At 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).

Literature
  • 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.
  • 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 imposter 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.
  • "Who Goes There??", the story that inspired The Thing (1982), used the same type of blood tests as the movie to identify the alien.
  • In Sunshine, when Sunshine and Con are being interrogated by the police, Con is exposed to sunlight as they suspect of him 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.
  • 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.
  • 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.
  • 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.
  • 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.

Live-Action TV
  • 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. Use of this test can be seen in the episode "The Adversary", among others. However, numerous problems with the test soon became apparent, such that a changeling could just kill some poor schlub and use their blood to pass the test. Ultimately, the person who proposed the testing was discovered to be a changeling infiltrator; he convinced Starfleet to use the test precisely because he knew that he and others of his kind could beat it. We even see one Klingon, who we later learned was a changeling the whole time, slit open his own hand.
    • 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.
  • In the Stargate SG-1 episode "Foothold", Melbourne cuts himself in front of Carter to prove he's human after an alien impostor is shown to have purple blood.
  • Supernatural:
    • 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.
    • 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.
    • In the seventh season, it's shown that leviathans have black blood. This trope is implemented when Frank and Dean have a small confrontation in which Dean cuts his arm to show Frank his blood. 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 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.
  • Fringe: The sinister cyborg shapeshifters have mercury for blood. Blood screening is standard procedure when shapeshifters are at large.
  • 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.
    • In an episode called "The Paradise Syndrome", Kirk plays along with aliens who think he's a god, until one of the aliens (who if memory serves were dressed like Native Americans of the Great Plains) cut him and contemptuously exclaims, "A god who bleeds!"
  • In the Doctor Who episode "Smith and Jones", the Judoon have scanners which 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.
  • Gaius Baltar spends most of the first season developing a Cylon detection test in the reimagined Battlestar Galactica.
  • 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.
  • 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.
  • 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 then a lousy player, and plays brilliantly. The Cat and Kryten then shoot him.
  • 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 what can identify them as imposters and spend lots of time on framing innocent people.
    1) Provacillium, a medication taken by the imposters.
    2) An eye test that analyzes proteins in the retina.
    3) Out-of-Character Alert. "I just remembered: Francie doesn't like coffee ice-cream."

Video Games
  • 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 or fake a death.
  • 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. This King is rather reluctant to do so.

Western Animation
  • 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!
  • 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.
  • 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.

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.
Identity ImpersonatorDoppelgängerImposter Forgot One Detail

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