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 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.
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.
- The Thing: 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.
- 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, 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.
- 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.
- 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.
- 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.
- The Bionic Woman and Gemini Man once shared a script about a lookalike for the title character infiltrating the agency where she/he works despite being ignorant of the main character's superhuman abilities. They are both assassins, targeting the main character's superior. At the climax, the hero(ine) and the double are both claiming to be the real deal; the hero(ine) proves his/her identity by using their special abilities—one by bionic-jumping to the top of a tree, the other by turning invisible.