I am probably guilty of sending "mixed signals" but I don't do it on purpose. I don't buy into the whole hard to get thing, I mean if I want to date you I'm going to at least talk to you... I think I may do this because I'm unsure of myself like you said. I mean I personally am not really able to tell if a girl likes me unless she is completely obvious about it (it's especially hard to tell when they're your friends).
Maybe just be a bit more obvious in your flirting and see if he notices and reciprocates. I mean I'm not going to go after a girl I think doesn't like me so keep that in mind.
For the same reason that girls send mixed signals - because we're human and we get nervous sometimes.
If the guy you like were totally up front, you'd get scared or possibly annoyed if he botched it.
Basically, if you're getting signals at all - he's into you on some level. Find out if it's just physical by bringing up sex. If he wants to talk about it, he has a physical attraction for you. If he steers the conversation back to something else or says he doesn't want to talk about that right now, you know he's getting to know you.
I believe guys tend to not play hard to get because seriously, why do we need to when the girl we desire so badly is right there waiting? Now for the mixed signals situation? Yes, we sometimes do that and it's usually because we aren't sure of your intentions and whether or not we have a shot. Us guys aren't able to understand ourselves at times, nevertheless you females.
In order to test if he likes you, go talk to him when he's ignoring you to his best capacity. Soften him up a little and show him you are harmless.