Fan Works / Bluff The Impostor

The Dark Lords of Nerima: When Ekim takes control of Ranma's body, with the intention of attacking Sailor Mercury, she intercepts him and, suspicious of his behavior, asks why he'd want to talk to her since she was the one who destroyed the "summoning artifact". Ekim responds that he began to respect her after that, which confirms her suspicion that it's someone else, since Ranma would know it was Sailor Moon who destroyed it.
Due to an earlier case of One Dialogue, Two Conversations, Ranma is convinced Mandy and her friends are his handlers in How I Learned to Love the Wild Horse but eventually notices that they're asking questions they should already have the answer to. He doesn't really get it until they ask about his sister, at which point he asks which school his sister goes to. They make a decent guess but if they were his handlers from WOOHP, they'd know he doesn't have a sister.^note
Infinity Train: Crown of Hydrangea: Grace is able to expose Dahlia by asking her about the rumors regarding the Aurorae Car; she nonchalantly claims that they're all true.
Vox Faunus: In A Tale of Two Queens, the heroes are already aware that the real Headmaster Theodore Berzins is currently in Atlas, and find themselves trying to ensure that his imposter doesn't realize they know about the deception.

All Assorted Animorphs AUs:
- In "What if Elfangor and Loren raised Tobias?", the Animorphs (wrongly) suspect that Tobias' parents are Controllers. Marco tries to get them to slip up by asking them about instant maple and ginger oatmeal and what they do in their spare time.
- In "What if Tom was infested by a member of the Yeerk Peace Movement?", it's mentioned that Marco spends over an hour asking Tom and Aftran questions in an attempt to prove that Tom has gone voluntary and teamed up with a Yeerk pretending to be Aftran as part of a plan to kill the Animorphs (he hasn't).
When Jake realises he's been sent back in time in What Tomorrow Brings, he tells Tom that he hopes to get his spot on the basketball team. Tom says he quit, confirming Jake's suspicions that he's still being controlled by Temrash 114.

In A Cure for Love. Light does this when Near calls him pretending to be L:
Light: L?
Near: Kira?
Light: Wouldn't you know if I am Kira or not?
Near: All voices sound very similar.
Light: You're not L.
Near: I assure you that—
Light: No. You're not. Where is L?
Near: This is L.
Light: Put L on or this call will end in record time.
L: Alright, Kira-kun. Since you can see past voice masks.
In We All Need A Hero Sayu Yagami (who became Kira instead of Light) pulls this trick on Mogi when he gives her a phone call while pretending to be L. Light had just returned from his encounter with the real L at To-Oh University and told Sayu all about it, so when she gets a call from another person claiming to be L she smells a rat. She tests the caller by mentioning how Light had wanted to kick off their joint freshman address instead of finishing it. Mogi responds by blaming the university, which confirms Sayu's suspicions that she's talking to an imposter L because Light had actually told her the exact opposite. She hangs up on him in short order.

In Harry Potter and the Goblet of Fire, we know that Barty Crouch Jr. killed his father after Crouch Sr. saw through his Polyjuice impersonation of Alastor Moody. Of Lacewing Flies and Treacle Tarts shows us why: Crouch Jr. addresses his father, and tells him he would've been a fair Ravenclaw student like himself. Crouch Sr. immediately points out Moody was actually a Gryffindor student when he attended Hogwarts.

In Chapter 21 of Stories and Tales from Dimension 63, Lisa uses this trick to determine if the "Lincoln" in her house is an imposter. She asks him how taking the Glow Serum (from the first episode of the series) she invented a while ago felt, and “Lincoln” answers it tasted like vinegar. The real Lincoln never took the serum (Lisa gave it to Luan after all) and thus shouldn’t know about this.

Principal Celestia Hunts the Undead: Celestia and Luna suspect that Sunset has been replaced by a changeling, even though she passed the Trust Password. Celestia struggles to think of something else to test her on, something small enough that a changeling wouldn't have been able to research it, but important enough that Sunset would never have forgotten it. Luna decides to come at it from a different angle—a lie that the real Sunset would never accidentally mistake for the truth. Then she walks up to "Sunset" and says she forgives her for having sex with Celestia. Since Celestia has basically adopted Sunset by this point, it's very obvious when the changeling tries to play it off.
Luna: [to Celestia] You're just mad because my plan worked.

Pokémon Mystery Dungeon: Pathways of Aspiration: Suspecting that somebody is impersonating her mother, Argon asks them why they named her that. Just as she suspected, the story her "mother" offers (that she was named after the chemical element that powered the lights in her father's guild) doesn't match up with the one she knows (that they named her that because they found it easy to pronounce, as well as ensuring her name would be called first in alphabetical order).

Distortions: While Hibiki's family is being moved to by some people who claim to be from SONG agents to safety, Hibiki's grandmother asks some questions about how Hibiki has been doing. Such as has the media circus been keeping her from getting enough sleep, or how she's been eating. Their responses — that the media has been keeping her up when Hibiki could sleep through a tanker truck exploding and eats three meals a day when she eats much more than that — tell her that they know nothing about Hibiki and aren't what they say they are.

Stars From Home: Scott uses this when he believes Raven is impersonating Alex; he is already convinced, but tries to prove it to Charles by having Alex describe the day Scott was born. It works. Alex appears several years older due to experiments keeping Scott young. So Alex couldn't remember the day Scott was born—because it happened before Alex himself was born.

Bluff The Impostor / Fan Works