Different Lies To Find The Spy

"I told Varys I was giving the princess to the Greyjoys. I told Littlefinger that I planned to wed her to Robin Arryn. I told no one that I was offering her to the Dornish, no one but you."

- Tyrion Lannister

Someone is a spy. But how to find them? This trope is about one solution. Tell multiple people different, but similar lies, and then see how the person the spy works for reacts. It may be worth noting that doing this doesn't inherently prove that the information leak is actively trying to act as a spy, maybe they just have Loose Lips and their spouse is the actual spy or something. In fiction, however, usually the leaker is a spy.

Compare Impostor-Exposing Test.

Fan Works

 * In A Young Girl's Delinquency Record, Jenny runs ran a small ring of drug dealers, and told Stevie that Dr. Brinkmeyer was the ring's supplier, but also told an unnamed redhead of a different unspecified supplier; it's safe to assume that more people were told different sources as the ring's supplier. After she accuses Stevie of telling the police who the ring's supplier is, Stevie makes a run for it. Jenny interprets the running as a confirmation that Stevie was an informant for the police (instead of there being a coincidence). When Jenny gets home, her knuckles are bloody.

Live-Action TV

 * Tyrion Lannister determined that Pycelle works for Cersei this way in Game of Thrones, as shown in the page quote.

Web Comics

 * Girl Genius has Gil successfully catching one spy for a group trying to capture or kill Tarvek this way here.

Web Original

 * Done in Fenspace during the "Boskone War", in order to root out which group in the allies was leaking secrets to the enemy. Each group was told to rendezvous at a different location; when the enemy forces showed up early at one location to ambush the allies, the allies knew where the leak was.

Western Animation

 * In the Bob's Burgers episode "Tina Tailor Soldier Spy", Tina Belcher told different girls in her scout troop where a lead was for a place to sell cookies to find out which of them was spying for Troop 257.

Real Life

 * Who leaked the spoiler that in Star Trek II: The Wrath of Khan? Thanks to somebody making sure each script had a unique code - in essence, something different was told to each person involved - we know that it was.
 * The Electronic Frontier Foundation asks, with good reason, "Is Your Printer Spying On You?"