"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."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 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. 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.
— R.J. MacReady, The Thing (1982)
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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.
- 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 imposter as only the person who placed them on another could remove them. It didn't work and they resort to Spot the Imposter tactics instead.
- 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 its her. Likewise, her manticore familiar can smell whether or not they are the real deal.
- The Thing (1982): Poking people's 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. 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 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).
- 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.
- 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.
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.
- 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 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 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. 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 Star Trek: Deep Space Nine, someone brings a few Tribbles from Kirk era and, given that they're Explosive Breeders...
- 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 Battlestar Galactica (2003). Unfortunately, his self-serving, cowardly, and at times down right 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.
- 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. 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 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."
- 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."
- 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 whose on the winning side being the real one.
- 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. This 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 it's 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.
- At a point in The Witcher 3: Wild Hunt, Geralt is offered a drink. Accepting has him then 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.
- 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.
- 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.
- Transformers Generation One:
- 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.
- 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.