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).
Carefully watch a persons eyes and/or body movement. Try to assess their normal behavior before you present them with the question/information. People will sometimes “freeze,” not quite facing the other person, and will move as little as possible. Alternatively, some people will fight this by being overly dramatic, moving the body much more than normal.
A lot of information is conveyed through eye contact; a brief glance at the floor or to the right says a lot. After talking in a relatively truthful manner, a person falls silent and immediately looks away, covering the mouth and fidgeting, are other signs.