There's a few things you can do.
1) Note his body language. Is he more relaxed, or more tense when you're around? Does you catch him looking at you a lot? There are a lot of indicators of affection, you just have to find them.
2) Does he seem to go out of his way to spend time with you?
3) A lot of times, when a guy is around a girl he likes, his voice will go softer than it normal. This is a big sign.
4) Hugs. Does he give you any? If the answer is yes, it's also a sign... Unless he hugs EVERYONE.
5) The touch barrier. This is the point where a guy feels comfortable touching a girl. For example, touching her arm during conversation. Generally, a sign of affection is about 4 to six times during conversation.
To let him know how YOU feel, just drop hints over, say, the period of a week. If he doesn't take the hints, then just come out and say it. When you do, though, try to get him away from his other friends. But, if you do, and he wants to be with you, keep it discreet. You can hold hands, be in the halls together, etc. etc., but don't go around screaming to all of your friends that you're with him. For me, and I'm sure many others on here, that's a massive no-no.
Hope this helped out.