You can tell if a guy you just met likes you pretty quickly, especially if he's shy. First off, chances are that if he came up and started to talked to you he likes you. Check if he stutters or looks away while talking to you. Also, look at what his hands are doing. If it seems he doesn't know what to do with them, then he is nervous and most likely likes you.
In a group of friends, he will try to stand out for you so you notice him. If his voice gets louder when you walk by or he tries to impress you are ways of guys standing out. Both most likely mean that a guy wants you attention. Realistically, if he likes you, he will look at you. When your eyes meet, smile and hold eye contact for a few seconds and he should come over and start talking to you. If he doesn't come over after prolonged eye contact then he is really shy. You can go over to him but he will most likely stutter all over the place and get all nervous. If he is that nervous then you mean a lot to him so he either really cares for you based on looks or he isn't very social with girls. Either way, if he can't get a sentence out, good luck with the conversation.