Derrick Parker, a 20-year veteran of the New York Police Department and co-author of Notorious C.O.P., says to look for physical clues, especially sweating and fidgeting.
2. Seek Detail
Liars' stories often lack detail, says Lindsay Moran, a former CIA officer and author of Blowing My Cover: My Life as a CIA Spy. Her solution: Push your subject for particulars. The more minutiae a liar has to provide, the more likely he is to slip up.
3. Beware Unpleasantness
"Liars are noticeably less cooperative than truth-tellers," found psychologists Bella M. DePaulo and Wendy L. Morris in a review of studies on deception. "Liars also make more negative statements and complaints than truth-tellers do, and they appear somewhat less friendly and pleasant," they write in The Detection of Deception in Forensic Contexts.
4. Observe Eye Contact
A subject's failure to make eye contact is often sign of deceit, say both former NYPD officer Parker and former CIA agent Moran.
5. Signs of Stress
Look for dilated pupils and a rise in vocal pitch. Psychologists DePaulo and Morris found that both phenomena were more common in liars than truth-tellers.
6. Listen for the Pause
Forced to make up a story on the spot, most speakers will take a beat or two to collect their thoughts.
7. Ask Again
Police interrogators often ask suspects to repeat their stories, and listen for inconsistencies to ferret out lies. But be careful: "Smart people maintain the consistency of lies better than dumb people," says psychologist Robert Feldman, a professor of psychology at the University of Massachusetts.
8. Beware Those Who Protest Too Much
Someone who consciously is trying to make you think he's honest--for instance, by injecting the phrase "to be honest"--may be lying. Most people assume they will be trusted most of the time. If someone expects otherwise, take a moment to ask yourself why.
9. Know Thyself
One reason liars succeed is that listeners don't really want to know the truth, says psychologist Feldman. So be honest with yourself about what it is you want to hear. You may wish to believe that a trusted employee didn't have his hand in the cookie jar. But does his story actually make sense?
10. Work on Your Intuition
"Good human lie detectors, if there are such persons, are likely to be good intuitive psychologists. They would figure out how a person might think or feel if lying in a particular situation, compared to telling the truth, then look for behavioral indications of those thoughts or feelings," write psychologists De Paulo and Morris.