Guys are usually straightforward when they speak. They don’t generally try to be cryptic or talk “around” a topic. They’re all business and logic, so if they are not lying, they are probably just telling it like it is. Don’t try to find a lot of subtext between the lines because there probably isn’t any.
If he says you’re not his type but he’s still interested, take it at face value. He most likely is trying to say that he’s usually not attracted to serious, brainy girls, but there’s just something about you that intrigues him.
If he’s just interested in sex, he most likely won’t mention you’re not his type, as that would work against his game. He’ll either lay on the charm and the old familiar lines, or he’ll say that you two should get together for some fun sometime.
He’ll say you’re “hot” instead of telling you you’re pretty. He’ll eyeball you up and down like an ice cream sundae, and he’ll move in close in an aggressive rather than tender manner. You’re a woman, so use your intuition. You’ll know what he’s really interested in, one way or the other.