My best advice to you is to ask him yourself
when his friends aren't around he talks to you and flirts with you. this means he likes you, but does not want to let his "popular" friends know.
the easiest way is to ask.
iam in love with this guy but i do not know if he likes me or some other girl
If a guy likes you but you don't know him he may look at you or walk by you whenever possible.
when he says he likes you.
http://www.wikihow.com/Know-if-a-Guy-Likes-You
The only way to know if a young man likes you is to go up to him when he is alone and perhaps ask him out to a popular cafe so you can get to know each other better.
Not sure how you expect someone to magically know something.
What do you do when a popular shy guy you know like likes you? Well, i think u should say something to him (eventually). As he walks down the hallway start by simply smiling. sit next 2 him in lunch, acknowledge he loves u without openy yelling it out that he likes u.
It is ok to like a guy after you know he likes you. That is perfectly normal and a part of life.
you really cant make someone like you but you can try to get to know him better and do the things he likes
You ask the guy.