How do you boys show affection with girls that you like?

lol, no. I try to be sweet, but I don't go too far. I try to be sweet to the point where I say the sort of things I don't imagine other guys she encounters would say. If he says something that a guy hasn't ever said to you before, on a complimentary level, either about your looks or personality, then it means he feels something special for you.
For example, if you've never had sex before, most "guys who want you in bed" would say "omg are you crazy! you're so hot though!". Instead, I'd say something that shows her I respect that and that she's being a smart girl. Then we'd probably go on to talk a bit about the realm of sexual ethics, which would be flirtatious of course.