always identical) will often ask the question: what differentiates these two outwardly identical people, and can others (in-universe) pick up on those differences? To answer, creators will sometimes have their characters administer the twin test.
The twin test is a quiz in which identical siblings ask other characters to point out which sibling is which. This can be done in a variety of ways, most commonly with the twins asking someone to identify them either face-to-face or using photos.
The twin test is often Played for Laughs, especially with Trickster Twins. If someone is particularly good at the test, the siblings may become obsessed with stumping them, escalating the difficulty of the test to absurd levels by dressing the same, adopting similar mannerisms, or even pretending to be the other twin. Single-Minded Twins giving a twin test are typically played for laughs with the joke being that the obvious lack of distinct personalities between the two will make answering correctly a matter of chance. The justifications a character gives for their answer, regardless of whether they were right or wrong, are ripe with comedic potential.
When Played for Drama, if most people fail the test, at least one of the siblings may become withdrawn or jaded about the nature of their own individuality. It may also jump-start a desire for one sibling to differentiate themselves as much as possible from the other and distancing themself from their sibling(s) in the process. Expect someone who consistently passes to be considered a True Companion or One True Love, since only someone who really gets the siblings can pass the test.
If siblings have Identical Twin ID Tag or experienced Twin Desynch, they are far less likely to give a twin test since the answer would be obvious (though savvy twins can exploit this). In more fantastical or action-oriented stories that include an Evil Twin, a twin test may be used to confuse or thwart The Hero and is likely to include raising the stakes of answering incorrectly into a Matter of Life and Death.
This trope also applies to Split Personality Tropes if one (or multiple) personalities asks a character if they can tell when a given personality is in control. Since typically no two personalities can be present at the same time (unless you're dealing with Literal Split Personalities), this is likely to happen over the long run, spanning multiple smaller tests.
Compare and contrast Spot the Imposter and Impostor-Exposing Test, which similarly focuses on a character correctly identifying another from a doppleganger. What separates this trope from those is that while the twin test can include one sibling pretending to be another, this is not fundamental and typically the twins present genuine representations of themselves. Similarly, in the case of split personalities, a twin test may involve Hyde Plays Jekyll, but it is not necessary for the trope.
- Buso Renkin: Papillion's Start of Darkness involves a particularly nasty take on this. After dealing with being The Unfavorite to his brother for years, on turning himself into a homunculus he made a deal with himself that he'd spare anyone in his family who said his name instead of his twin's. There were no survivors.
- Cipher: The main plot is the female lead Anise being challenged to figure out who is who between the twins Siva and Cipher. They promise to tell her why they have the habit of making Twin Switches if she figures it out.
- Mars (1996): Whenever an old photo featuring him and Sei pops up, Rei will ask Kira which one she thinks is him. Each time, Kira answers correctly and while doing so she is depicted with some form of Love Bubbles, indicating that her ability to pick him out is meaningful to Rei
- The One: Downplayed photo version. Lele being captivated by a portrait of Angus Lanson and falling for him jump starts the entire plot. She later finds out however that it's actually a portrait of older twin brother Eros, who had to fill in for his brother the day of the shoot. This is heavily foreshadowed from the outset, as when Lele literally falls into Angus's lap during a fashion show early in the story, she is taken aback by the different vibe she gets from him in person as compared to the photo. And when she is with Eros, she is consistently reminded of the portrait. Given that they end up falling in love, this is used to indicate that Lele understands and is "the one" for Eros. Eros knows she is mistaken but chooses not to tell her, letting her find out on her own.
- Ouran High School Host Club: Plays the trope both for laughs and for drama. Hikaru and Kaoru Hitachiin play the "Which One is Hikaru?" game, both as a form of entertainment for themselves and others and as a defense mechanism to keep people from getting too close to them, mocking and ridiculing those who fail the test when administered outside of club activities. Most people DO fail, including their parents. While Tamaki fails too (eventually getting it right by nothing but a blatant lucky guess), the fact that he actually cares which is which is why they become friends with him. It's a shock to both of them that Haruhi passes the test, every single time. Haruhi consistently passing makes both Hikaru and Kaoru develop romantic feelings for her later on.
- The Quintessential Quintuplets: In one of the series' recurring themes, the quints tell Fuutarou several times that if he loves them he'll be able to tell them apart. This leads to them giving Fuutarou many twin tests over the course of the series, with Fuutarou even invoking the trope himself by asking them to test him. Naturally, Fuutarou fails each test in spectacular fashion. By the end of the series he's engaged to one of the sisters and as a final test of Fuutarou's love for her, the sisters pose as identical brides and ask Fuutarou to identify his fiancee. He goes a step further and correctly identifies all five, showing he has learned to love all of them.
- Doubly subverted in Harry Potter and the Philosopher's Stone, in the scene in which Harry goes to Platform 9¾ for the first time. George and Fred Weasley bait their mom Molly into a twin test after she correctly calls out to Fred. They trick her into thinking she called out to the wrong twin, saying that as their mother she should be able to correctly identify her own children, only for Fred to reveal immediately after that she was right the first time.
- The Parent Trap (1998): Hallie and Annie deploy this trope on the fly towards the end of the movie to confuse their parents, who are trying to leave with the child they are responsible for. They argue that if the parents can't tell who's who, then they can't split them up, otherwise they'll just have this same issue all over again.
- The Prestige: A very relevant and crucial aversion. The key to Borden's version of the Transported Man trick is that "he" is actually two twin brothers who have decided to live their entire lives as one man for the sake of their magic acts, down to lying to "his" wife about it. This has disastrous effects as the constant gaslighting by Borden about his abrupt behavior changes and obvious infidelity destroys his marriage and causes his wife to commit suicide.
- Serenity: When Mal meets with his contacts Fanty and Mingo, they try to pull this on him by claiming they're the other one. Mal sees through it, and when they ask "How do you always know?" he replies "Fanty's prettier." The novelization explains that Mal has identified certain tells— Mingo has a slightly deeper voice, for instance— but refuses to tell the twins what they are.
- In Lottie and Lisa, the titular twins put on identical dresses and wear their hair the same way for a day at summer camp. They then challenge the other campers to figure out who is who. No one manages it by looking at them, but since their personalities are very different, one girl has the idea to pull the hair of one of the twins. When this earns her a slap, everyone knows it was wild-tempered Lisa and not the shy Lottie.
- In Poor Miss Finch by Wilkie Collins, the title character Lucilla Finch (who's blind) is challenged to tell the difference between her fiance Oscar and his twin brother Nugent. This becomes a crucial plot point later in the story when Nugent tries to marry Lucilla under the guise that he's actually Oscar. She finds she can do it by holding their hands:
Lucilla: "When Oscar takes it, a delicious tingle runs from his hand into mine, and steals all over me. I can't describe it any better than that."
Madame Pratolungo: "I understand. And when Nugent takes your hand, what do you feel?"
- In Sophies Adventures, Sophie's older brothers, who are twins named Matthew and Mark like to ask people if they know who is who, and claim the person guessed wrong even if they guessed right.
- Full House: Played for Laughs when Jesse's Greek relatives visit. This is an interesting case that Makes Use Of The Twin in real life to play a character that is not the twin of the character they are identical to, but is just a nearly identical cousin. Joey dresses youngest child Michelle, played by either Mary-Kate and Ashley Olsen, and Jesse's cousin, played by the other Olsen twin, alike and then challenges Danny, Michelle's father, to guess who's who. The jig is up in two seconds though:
Joey: Check this out! Try to tell 'em apart!One of the girls: You'll never guess, Daddy!Danny: Ohhh this is a toughie, but I bet that the real Michelle...is the one that just called me Daddy. And you (to other girl) are Melina.Michelle: Aw, nuts.
- Downplayed and subverted in the season 2 episode of Sister, Sister, "Joey's Choice." After the same boy, Joey, accidentally asks them both out thinking they were the same person, Tia and Tamera reluctantly share a date with him. Though Joey doesn't mind, the girls do and at the end of the date Tamera asks him to choose between the two. To her surprise, he picks Tia, who at this point in the series has been characterized as the less outgoing and more boy shy of the two. Tamera asks him why and Joey admits that because he didn't really know why himself, he based his decision on the "stupid reason" that he liked the shoes Tia had on. Tamera leaves only to remember that Tia is wearing her shoes.
- Inverted in the season 1 episode of The Suite Life of Zack & Cody, "Poor Little Rich Girl." Zack and Cody watch a baby video of theirs in which their parents accidentally mix the two up. Despite their mom pointing out that all babies kind of look alike anyways, the two spend the rest of the episode having Identity Breakdowns, questioning their own identities, and adopting the traits of the other. They demand their mom find their birth certificates so they can prove once and for all they are who they thought they were.
- The Ricky Gervais Show inverts the trope when Karl is asked what he would do if he had a doppelganger who looked, thought, and acted exactly like him. His response leaves Ricky flabbergasted.
Karl Pilkington: How would I know which one I was?
- Love & Pies: When Joe arrives to see his identical twin brother Sam impersonating him to date the former's Love Interest, Amelia, Sam claims that he's the real Joe while Sam was out at the baker's retreat. Amelia then quizzes them on where they have been for the past three days, and the real Joe proves himself by showing her the prize he won at the retreat.
- In Tawawa on Monday The Twins regularly make their neighbor and shared love interest try to tell them apart. How does the neighbor know which is which? By looking at the twins' panties and bras, of course! Which makes passing these tests difficult to do in public since it requires him to expose The Twins.
- DuckTales (1987): Mrs. Beakley immediately proves her mettle as the triplets' nanny by not only not being put off by their pranks but by quickly determining they'd pulled a Triplet Switch, without ever having met them before or having any logical way to tell one from the other (sure, they wear different colored clothing, but she shouldn't have known which brother favored which color).