Pamela Meyer, author of Liespotting: Proven Techniques to Detect Deception, with some tips on how to spot a liar:
Baseline your subject: Obtain a reference point for measuring changes later. Have a normal conversation with someone and note how they typically move their hands and feet, what their posture is, the style and duration of their laugh, and their fidget patterns. Later in your interview, note which questions create significant shifts from this baseline. Nonverbal clues can be revealing because liars don’t rehearse gestures, just words. They freeze their upper body, oftentimes look down, lower their voice, slow their breathing and blink rate, slump, and then exhibit relief when the interview is over. Interrogators will often end an interview prematurely just to look for that relief—that shift in posture and relaxation. Pay attention to science and not myths: We think liars won’t look you in the eyes, but it turns out an honest person will only look you in the eyes about 60 percent of the time.
An easy verbal cue to watch for is people resorting to formal, rather than relaxed, language. For example, the “noncontracted denial”: “I did not have sexual relations with that woman, Miss Lewinsky,” rather than “I didn’t ….” They may tell their stories in perfect chronological order. Try to get them to tell their story backwards! They can’t do it. Honest people remember stories in the order of emotional prominence, but liars will recount a story in chronological order. Memory rarely works that way. Liars may also use past or present tense inappropriately. Scott Peterson famously self-corrected when interviewed by Diane Sawyer about the murder of his pregnant wife. He said: “She was amazing … I mean she is amazing.” He signaled to interrogators that he thought she was dead.