Well in situations like you wrote of, I usually just smile (feel a bit flattered) and move on with my life if I am not interested. If I am interested I might strike up a conversation. As for girls hitting on me, once again it depends on the situation. If I am interested I will strike up a conversation, if not I'll usually converse with the girl but she has to lead the conversation (unless I'm busy with something then I just make it known that I am in the middle of something).
Honestly the guy you spoke of sounds like a jerk, but it could just be how you perceived the situation. You sound a bit self conscious (just a guess from the way you write about yourself, though you are confident enough to just check a guy out if you want to so kudos), so it may not be that he was laughing at the thought of you checking him out as much as it was him feeling a bit awkward at the situation (which isn't that uncommon for guys).
Honestly the guy you spoke of sounds like a jerk, but it could just be how you perceived the situation. You sound a bit self conscious (just a guess from the way you write about yourself, though you are confident enough to just check a guy out if you want to so kudos), so it may not be that he was laughing at the thought of you checking him out as much as it was him feeling a bit awkward at the situation (which isn't that uncommon for guys).