They say that the eyes are the windows of the soul. When someone can't look you in the eye, they may well be lying. Or they might have something wrong with their eyes. On the other hand, you don't want to make too much eye contact after a little fib. Staring into someone's eyes like a crazed bush baby is a dead giveaway.
Some people suffer from involuntary or pathological lying.
Ask them where they're from, and they'll say New Guinea, even though you know they're born and bred in Huddersfield. And then they'll make up a name that sounds suspiciously like a mispronounced suburb of Huddersfield. When you think someone's lying, there are two ways of dealing with it. Best not to say this during the wedding service though.