One indicator of a liar is excessive gesturing and fiddling with objects while telling the lie. If you ask someone a question and he starts bouncing a ball or becoming very animated with his hand gestures, it may be a sign that he's lying to you.
Liars who don't exhibit excessive gesturing may try to exert control over their body movements by placing their palms down while telling the lie. Exposing the palms is a sign of vulnerability, and hiding them in the midst of a lie is usually an unconscious gesture.
A person might deal with the tension of telling a lie by wrapping his ankle around the leg of the chair while telling the lie. This is likely a subconscious gesture of restraint and an indication that he is holding something back -- likely the truth.
The body releases adrenaline during a lie, as it is usually a tense moment. In the absence of any physical indicators, a reliable sign that a person could be lying to you is if his pupils suddenly dilate. The sudden release of hormones causes this involuntary response, which is a likely indication that you're not getting the truth from this person.
Watch his hands right after he makes a declarative statement. If a hand goes to the mouth, he could be lying. A person who doesn't typically lie will exhibit this gesture, literally unable to believe that he just told a lie.
Rather than playing with an object, someone in the process of a lie might place an object between you and him as a defensive measure. If you observe this behavior, look carefully for other indicators that might confirm your suspicions about the validity of what you're hearing.
Watch the nose to see if it wrinkles. A liar literally may not like the smell of what he's dishing out.