Lies are hard to come up with on the spot, even for seasoned liars. If you suspect someone is feeding you a false story, draw it out by making them think too much about their story. Photo by Alan Cleaver.
As tips blog Barking Up the Wrong Tree explains, a liar naturally wants to keep a lie simple. The fewer details they have to come up with on the spot that are all consistent with each other, the easier it is to be convincing. However, you can counter this by asking open-ended questions that increase the cognitive stress on the person you suspect is lying. The more you have to make them think, the less likely they are to tell a straight story:
So if you want to make a liar reveal themselves, you want to increase their cognitive load. The more they have to think, the more likely they are to make a mistake.
How can you do this? Police detectives ask open-ended questions that make them keep talking. Unexpected questions they're not prepared for are the best. Anything that mentally exhausts someone is good.
The more you can keep them talking, the more likely they are to trip up. Open-ended questions are the best for keeping them talking (as opposed to simple yes or no questions), because they force the person to come up with details on their own. The more details you have, the easier it is to find an inconsistency.
How To Tell If Someone Is Lying: 5 Research-Backed Secrets [Barking Up the Wrong Tree]