How do you know he is lieing?

how you know if he is lying well if you question him on it and he cant look at you or he's looking off to the side the one indication of a lie, also if he is always accusing you of things you didnt do like cheating or something he may be trying to shine the light off of him so he wont get found out about what he is doing, also if you question him and he gets mad and doesnt want to answer the question that another sign of a lie because if its not true why cant he just answer the question???