Hey, those are some really good questions... sorry, I'm not a guy, but I'll try to help you out!
All guys are different. Some of my guy friends are really confident and they make it obvious that they like the girl and they ask her out. But, I think it's a little more than coincidence that all of my guy friends who do this are either seniors in high school or undergrads in college. As a general rule, older guys usually let you know if they like you, of course, this is not the case with all guys.
Some guys act really shy around the girl. They rarely talk to her, and if they do, they will look at the ground. Sounds weird, but that's what they do. With these guys, you're going to have to be the one making the extra effort to talk to them... but don't be too aggressive... that'll make them uncomfortable.
Some guys will just completely not even talk to her, but just stare at her and avoid her. http://www.healthboards.com/ubb/rolleyes.gif
As for being able to tell if a guy likes you... that gets a little more complicated. With the guy I like, I was able to tell because every time I would try to make eye contact with him, he'd look at the ground, and he was always too shy to talk to me, but he was always really talkative with all of the other girls.
Every guy is different, so they all have different ways of letting you know that they like you.
Everyone drops hints when they like someone. A lot of my guy friends watch them from a distance or out of the corner of their eye to see if they girl they like is noticing them.
When I'm not sure if a guy likes me, I usually try to get to know them and befriend them. A hint could be if you give them a big smile, and if they light up... that's usually a good sign... but that doesn't necessarily mean that they like you.
I think the first step is just trying to talk to the guy, become friends... it might grow. Also, if you're friends, it is a lot easier to ask them questions about what they're looking for in a girl, etc...
Hope that helps!!