They cannot look you in the eye , usually looking up during the conversation. Body language is not their norm. They try to turn it around on you by getting angry at you or accusing you of something. If your gut is screaming lie, it probably is.
Lots of people have different signs that convey that they're lying. Watch someone talk when you KNOW they're telling the truth, then compare they way they act/talk when you have doubts about whether they're fibbing.