For most guys, it's that they're paying lots of extra attention to you in some way.
- He smiles at you.
- He stares at you all the time, even if he turns away when you look so he doesn't get "caught."
- He goes out of his way to talk to you, even crossing the room to do so.
- He tries to stay physically close to you; sitting next to you, standing beside you, etc.
- He touches you (hand/arm/shoulder/small-of-back, etc.) when he usually doesn't with people.
- He does things for you: gets you coffee, carries in your groceries, buys or makes you things, etc.
Some guys are too shy to do much more than stare, but it shouldn't be too hard to tell that they're paying you noticeably more attention than they pay to other people, especially other girls.
Now, what's even more important for you to know is that they are doing all of this so that you'll notice that he's interested in you, and what HE is looking for is some indications from YOU that YOU are interested in HIM.
- Do you smile back?
- Do you go out of your way to talk to him or include him in conversations?
- Do you get physically close to him, or touch him?
Guys who are giving you "non-verbal signs of attraction" are guys who aren't bold enough just to come right out and ask you out, or to verbalize their attraction for you, so they are trying to tell you, non-verbally, that "hey, I like you." If you don't give them a positive response, either verbally or through your own actions and body language, then the guy will assume that you aren't interested, and he'll never make a move and will probably just fade away before too long.
From the guy's point of view, he doesn't want to risk outright rejection, so he's "testing the waters" to see if you have some interest in him before he asks you. If he doesn't see any, and if it isn't CLEAR to him that it IS interest, then he'll assume there isn't any, and will move on to another girl so he doesn't have to hear an ego-crushing "no" from you. That's why, if you like a guy, and he's showing interest in you, it's SO important that you show it back. Remember, too, that he's already afraid you don't like him, and is likely to need some solid proof that you do. If he's unsure about your body language, then he'll default back to his position of "she could never like someone like me." So you need to make your responses crystal clear and obvious, so that he can have no doubt.