Usually by their gestures and body language. If you lock eyes with each other and he can't hold the stare, he likes you. I think there is an aura  around a person if they really like you, it is so obvious it can't be ignored. But it is difficult to recognize if he Loves or Likes you, that is something you have to differentiate. 

If he contains his emotions and trying to be tough all the time, then he's probably the opposite (he's the sensitive type of guy), but insecure to show it because he doesn't want to get hurt.