One way of telling if someone is not telling you the whole truth is to watch out for their hand going up to their nose.
If you notice the hand move up and touch the side of the nose it is a strong indication that they are either lying to you or that they are exaggerating what they are telling you.
The theory behind the hand to nose gesture is to mimic the person covering their mouth as if they are telling lies. Children will often bring their hand to their mouth to cover up a lie they may have told. The idea behind touching the nose is the grown up version of covering their mouth!