Some signs that a person might be lying include quick changes in head position, heavy breathing, limited body movement and the frequent repeating of words or phrases. When attempting to determine if someone is lying, it's important to consider how they normally act and look for subtle differences in behavior.
A person who is lying may make sudden movements of their head when asked a direct question. They may retract their head, bow it down or cock it to the side, often right before they respond to a question. Someone who is lying may also begin to breathe heavily. The liar's shoulders may also rise and their voice may become shallow. These physical changes occur due to fluctuations in heart rate and blood flow. These are reflex actions that occur when the body is feeling tense.
While many people twitch when they are nervous, in many instances a liar may not move at all. People typically make many sorts of subtle, unconscious movements when conversing naturally. The absence of these movements may demonstrate a biological "fight" response, in which the body readies itself for confrontation. Finally, a liar may frequently repeat certain words or phrases. This is an indication of the individual trying to convince themselves of their own lie through repetition. This process also gives them more time to gather their thoughts and keep their story straight.