You actually can tell with girls and the tone of their voice. I go out of my way to not indicate in any way to this guy or others around him that I am hopelessly into him but I've been called out on it as "your voice goes up like ten octaves when you talk to him!"
No I kind of suck at this. I assume but I'm never sure so I just shrug it off. There are times when it is pretty obvious that a guy is into by the way he looks at me but it's easier to tell when I'm not interested in the guy. When I'm into a guy, I feel like I'm just imagining that he's looking at me weird. It's kind of difficult to see things the way they really are.
No idea, but it seems like you can intuit it. I felt it yesterday. Some girl was beaming at me for 10 seconds straight. Perhaps she was just being polite. Notice how you sense something, and then your mind takes over and says, surely not, don't reciprocate, you'll make a fool of yourself. What would happen if you went with your initial instinct?
You sir have a magic gift most men wish they had haha. I can see women being able to pinpoint out which guys like them because guys tend to show it more than women, but for a guy to be able to say that...Wow!