Smile

Guys have no idea, but you can absolutely read what they are thinking right on their face. When a guy has a crush on a girl, he will smile the minute he see's her, and the smile will stay on his face until long after the girl is gone. If you catch a guy looking, he will look away the minute you make eye contact, but that small secret smile will still remain on his lips. There is no mistaking it, you absolutely know what he was thinking.

On the other hand, if a guy is angry with you, he will avoid looking at you at all cost. He will say whatever you want him to say just to get you as far away as possible. Why, because he knows that the minute he looks at you, that smile will spring forth, and he will be unable to be mad at you any longer.