Most Helpful Guy
If he thinks she knows he likes her :
-Avoidance (Won't make eye contact, might walk away from her)
Unfortunately I can't really think about anything else, but the main thing to focus on is the person's friends. Friends are a sure giveaway as to whether he likes you or not.
If he doesn't know that she knows that he likes her, just look for things out of the ordinary. He might approach you out of the blue, or even talk more often than usual. When I had a crush on a girl you could tell because I became social.
Anyway, the main thing is the friends. Pay attention to his friends, and if you really want to 'yolo' it, you could even ask them. Most of the times they'll spoil it without you having to do that.