How to tell if someone is lying to you

The first question is: Do you really want to know? Some people willingly blind themselves and accept a lie as fact solely because the truth might be too painful to handle.

The second question is: Why would this person lie to you? What is at stake here? Maybe he is afraid of being punished, or of the loss of the ability to obtain what he desires (for instance, your business), or, ironically, he is afraid he will lose your trust.

Know how he talks normally. The first step to reading a liar's body language is to know his normal, everyday conversational habits.

Notice how stress influences his patterns. It is notoriously difficult to tell the difference between a person who has been put on the spot, and an out-and-out liar.

Look for the signs. Here's list of the most common “tells” people have while attempting to lie.
Little to no body movement. People will sometimes “freeze,” not quite facing the other person, and will move as little as possible during the lie. Alternatively, some people will fight this by being overly dramatic, moving the body much more than normal to try and “sell” the lie.
No eye contact. A lot of information is conveyed through eye contact, and most people have an extremely hard time lying to someone while looking directly into his eyes.
Stress gestures (rapid blinking, scratching, itching, swallowing, fidgeting, etc.).
Looking up to the right. Looking up and to the right stimulates the part of the brain associated with imagination (that is, making things up), whereas looking up and to the left stimulates the part of the brain associated with recalling memories (that is, telling the truth).
Touching the nose and covering the face/mouth. Habits from childhood, these gestures are a subconscious attempt to “cover up” the lie.
Rise in vocal pitch.
Wide-eyed, innocent look.
A noticeable pause in speaking. Most people lack the ability to come up with a plausible fiction in an instant, so they have to take a moment to think something up.

Lying by omission. Sometimes a lie will happen by not speaking – that is, the liar omits an important piece of information from an otherwise truthful statement.

Ask. A risky maneuver, as a person who is telling the truth may be horribly offended. Sometimes, however, a liar will come clean if given the chance to, as he realizes that continuing to lie when you know he's doing so would be far worse than just admitting the lie.

Trust your intuition. Notice I say “intuition” and not “desire.” Most people have a pretty decent “lie detector” built in, and if you trust it, it can usually point the way.

...If it feels like a lie, it probably is.

From here