Men are less equipped with a range in body language. One signal to watch out for is when the guy pushes his shoulders back and directs his breath to his chest. Another strange one is when a guy puts his fingers through his belt loops like a cowboy.
Watch for eye contact.
Eye contact is the universal body language clue that can communicate so much. Both men and women will use eye contact when they like someone. Try not to hold contact for too long, or else you might ruin the mystique around it. The biggest give away is if he looks at you and you both hold eye contact for a moment and suddenly look away.