You don't. Step up and ask or make a move. You're never gonna know unless you do something. Maybe asking straight up wont work. You should try flirting. But don't act slutty cause that will make any honest guy move away. Just try flirting slightly. Tell him he's cute, if he gets shy and all, he probably likes you. But If he doesn't, if he just says ok and doesn't really bother about it, then he doesn't like you. Just try something. Maybe get a friend of his and yours to ask him if he likes you or write him a letter. A little note and start of like that, writing secret admirer notes to each other. And then when you're sure of it, finally reveal who you are to him and y'all can be together if y'all like each other.
To know if a guy still likes you when he's too shy to talk to you can try talk to him online or via phone. This is because most shy guys tend to open up more when charting with someone not physically. Another thing is that you will notice him checking you out a couple of times.