PsyBlog has an intriguing piece about why detecing whether someone is lying is so difficult for most people, and what you can do about it. From the piece:
So, why is it so difficult to tell when people are lying? That's what Hartwig & Bond (2011) examine in a meta-analysis of dozens of studies. They looked at all kinds of different cues to lying like fidgeting, postural shifts, head movements, gaze aversion and speech rate.
What they found is that overall people do pay attention to many of the correct cues to lying. These include things like:
Vocal immediacy, i.e. the extent to which someone replies directly to questions. The vaguer someone is, the more likely they are to be lying.
Indifference: if the speaker seems unconcerned then this is associated with lying. It's probably because they're trying to play it cool.
Thinking hard: lying is hard work so when a person has to think hard about a question, it might indicate they are lying.
Being uncooperative: pretty obvious, but still being uncooperative is often a cue that someone is trying to conceal something.
Read the entire piece at PsyBlog.