Former FBI profiler Noah Boyd (not his real name), author of The Bricklayer, shares how to spot a liar:

"In my experience, there are no standardized 'tells' that prove you're being lied to. What the investigator tries to do is find, or cause, anomalies - those tiny deviations from the norm that indicate that a person's veracity is suspect. Most commonly used is the examination of the information given, but it can also be a physical manifestation, such as a tic of the inidividual's voice, a facial clue, or even body movement. I was once able to help solve a kidnapping/homicide because my experience told me that the person I was interviewing was being too polite. Each of these items listed is not soley a proof of deception, but if one is spotted, you may want to look for others. It's generally thought that these observable variations are caused by an emotional reaction to lying, and, even worse, being caught: 

1. Stuttering/slip of the tongue

The mind is distracted with creating the next lie, or considering the fragility of the one just told. 

2. Hesitation before answering 

This means that the person is considering any flaws in the deception about to be offered. 

3. Forced facial expressions, such as smiling too long 

Done in hopes of convincing the other party of their lack of worry.

4. Change in the rate of blinking of in the pitch of one's voice

This is an uncontrollable reaction to guilt or the worry of being discovered. 

5. Eyes that divert - no lasting eye contact 

This indicates worry that the other party is going to pick up a "window to the soul" signal.

6. Increased hand activity, like fiddling with objects or their fingers 

This is emotion manifesting itself as a phyical need to relieve the stress of lying. 

7. Sitting on their hands to stop their fiddling

They've realized this is a tell, and they are trying to stop it. 

(From Guyism)