Offering The Killer His Own Poison
All the suspects in a case of poisoning are offered some form of food and drink. The killer refuses because he knows it's poisoned.
A subtrope of I Never Said It Was Poison, specific to literal cases of poisoning. The police (or, more likely, a clever Private Detective or Amateur Sleuth) offers some item of food or drink to all the suspects of a case of murder-by-poison. The source of the poison hasn't been made public. They either say up-front that it was the same item that poisoned the victim, and arrest the person who refuses to eat/drink, or they wait 'till everybody's already started, and THEN do the reveal - and arrest the person who starts gagging and grabbing her throat. In either case, it works the same way - only the killer would know that it was the coffee/tea/chili/truffles/whatever that was poisoned. And of course, since the police already figured it out, it's not ACTUALLY the poison-laced product... A variant involved having a Corrupt Corporate Executive drink water from a heavily polluted site that he's insisted was perfectly clean when the locals started complaining about falling over with blue faces. Same deal - the fact that he immediately heads to the hospital to have his stomach pumped proves that he DID know about the pollution.
- An episode of Bones combines this with The "Fun" in "Funeral" when a man who was apparently killed by an aneurism in the brain, turns out to have been killed by poisoned tea. The team discover it too late to run their usual forensic wizardry, so they instead offer all the mourners at the funeral a nice cup of tea...
- An episode of The Mentalist uses it - a chef is poisoned during a Cooking Duel (by Gin - he was a relapsing alcoholic), and shortly afterwards, ANOTHER chef drops dead from the same poison, from a different source. At a subsequent memorial dinner, Jane doses the entree with the second victim's chili-powder (having deduced that this was the vector), and reveals the killer.
Hello, Unknown Troper. You'll need to get known to lend a hand here.