There isn’t one reliable behavioral cue that consistently reveals that a person isn’t telling you the truth, but the most expected sign of dishonesty is evasiveness.
Does the other person evade answering direct questions or declare, “I don’t know,” “that’s about it,” or “I don’t remember doing that?”
Instead of making direct denials, do they seem to have been caught off guard and take more time to think up a believable response?
Idea for Impact: To detect a lie, listen and pay attention. If lying is nothing more than communicating false information, dwell on what’s being said. Does it make sense? Does it align with other facts you’ve mustered or anecdotes you’ve heard? Do the answers to your probing questions stand up to scrutiny? Does the story begin to shift?