Stiff body: The person who lies unconsciously thinks that he could be unnoticed if he didn't move, thats why his body may be stiff with almost no movements
Deviation from base line behavior: Everyone of us has got a certain natural way of talking, walking and answering questions, this is called the baseline behavior. when people lie they usually deviate from this baseline behavior, thats why you can tell someone is lying by comparing his current behavior with his baseline behavior.
Anxiety: Ask the person directly if he was lying and watch for signs of anxiety that may appear on his body language (sweating, fidgeting or nail bitting). The person who lies may become anxious when confronted.
Blinking rate: Its believed that your blinking rate increase when you become anxious, just notice the person's blinking rate when being confronted and then compare it to his normal rate.
speech errors: We all make mistakes but when speech errors increase without an obvious reason then the person may be telling a lie. These speech errors happen as a result of the resistance that arises between the person's desire to be right and his need to lie.
Paying extra attention to their words: When a liar is asked whether he is lying or not he will pay extra attention to what he is saying while replying to the question not to get caught and as a result he may speak slowly,hesitate and become anxious