The eyes are the major signs that show someone is lying, if the person is lying then they will avoid eye contact. Observe the body language such as pressing fingers to lips or playing with hair. Watch for personal distance and over-rehearsal.
It's really just if what they say makes sense or not. Also, your history with that person and if you trust them. You can't tell through body language. Some people are shy or if you have an expression like you don't believe them, they may look guilty because of self-consciousness.
If they look away or can't look you in the eyes while telling you whatever it is there saying. Also there voice can change and get shaky at times. They trip up on there words. Online if there really "braggy" about it. Like constantly repeating it and if you say something like "oh thats cool" and they come back with "I swear!" and stuff like that. Its really easy to tell and they most likely will trip up at least once whether its big or little.