The traditional wisdom that states you can tell a person is lying by looking in their eyes may have some merit to it. While it cannot work as a lie detector, which is effective 100 percent of the time, eye movement is closely connected with cognition and thought.
When a right-handed person looks up and to the left, this is indicative of their making a visually constructed mental image. Up and to the right indicates a visually remembered image.
Straight left is connected to auditory construction, and straight right to auditory memory. Left and down is often indicative of memories of smells, feelings, or tastes, and right and down often represents internal dialogue.
So, for example, if someone glances to the left as they answer a question, they may be making up the answer, whereas a glance to the right means they are remembering it.
Remember that it's not a sure-fire method, though ... and also that most left-handed people will have the opposite meanings for their eye-directions.