Liars are more likely to ask that questions be repeated and preface pronouncements with, "to tell you the truth," and "to be perfectly honest," says Buckley. Evasive answers to direct questions should raise your hackles, too.
Liars--amateur ones, anyway--may not have thought through all the particulars of their stories. If you suspect you're being lied to, gently probe for details. (You don't want the person to know you're on to him.)
When a person is lying, the gaps between their words often increase, according to a 2002 study led by Robin Lickley, professor of speech and language at Queen Margaret University in Edinburgh, Scotland. While honest folks have the truth locked and loaded, liars tend to take more time between points--no doubt searching for which approach will be the most convincing.
Used-car salesmen notwithstanding, people generally don't like to lie. It makes them uncomfortable, even surly. "While a truthful person is concerned, composed and sincere, a liar is often defensive, guarded and less cooperative," says Buckley.
When honest people tell stories, they may realize partway through that they left out some details and unselfconsciously backtrack to fill in holes. They also may realize a previous statement wasn't quite right, and go back and explain further. Liars, on the other hand, "are worried that someone might catch them in a lie and are reluctant to admit to such ordinary imperfections," says psychologist Bella DePaulo, author of more than a dozen deception studies.