Some people think liars try to avoid eye contact, but this isn't necessarily true. Deliberate liars know to keep their gaze fixed to help support their point. Casual liars find it more difficult to look into a scrutinizing face. Sometimes a liar's pupils widen, depending on how stressful the situation is. Others may appear as though they are reading from a script in their head, by moving their eyes back and forth or rolling them.
Vocal inflection tends to change when someone is lying. Listen for inconsistencies compared with her normal speech. If her voice goes up in pitch at the end of sentences and that is abnormal, she may be deceiving you.
Closed-off and defensive body language is a sign that someone is being deceptive. Crossed arms are more suspicious than arms that hang casually at the side or are used appropriately within the context of the story. If someone is not facing you squarely, he may be hiding something. Facing you square on is more indicative of honesty. Similarly, someone who faces you straight on when seated is more likely being honest than someone who is turned to the side or sitting on one hip or the other.
Attention to Detail
Liars tend to give you too much detail in an effort to make their story seem more legitimate. If you find yourself feeling that what a person is saying is extra and unnecessary, he may be trying to put you at ease with extra information.
People who are deceptive make a few constant language choices that may help you spot them. Liars tend not to use contractions such as "don't," "can't" or "it's." They tend to use more negative words, stemming from the subconscious guilt they feel. They also use fewer first person pronouns in an effort to distance themselves from the story they are telling.
A good way to catch a liar is to test her logical process. If she can't keep her story straight and logical, she likely will slip back into vague statements and claims such as "I cannot remember exactly." Ask for details about the story and then ask the person to repeat it so you can listen for inconsistencies.
A good test of deception is to determine if the person has a motive to lie. Other than compulsive liars, people generally won't lie when they have no reason to. Do they have something to gain by lying? They could be saving your feelings or keeping your affections. If they lack a reasonable motive for lying, they probably aren't. If they have motive, the odds are much greater that they are lying.