I can't speak for the other guys, but whenever I see a couple, a girl smiling, when I'm with my friends or everything that makes me feel happy I think about my girl, I wonder what she's doing at that moment, if she is happy etc.
Don't worry, probably that guy is thinking about you right now ;)
you can't know if he does or doesnt. but I'm assuring you he thinks about you at least once. Ill admit that even as a girl I think about guys alll the time, every one I pass on the street, its pretty weird but TRUE, and I'm assuring you its the Same way with guys. haha and I'm not even a guy but I know its true