Well, just to make this clear and accurate, I am a girl.
That said, let me continue.
Although it depends on your relstionship - how well you communicate, "mesh", 'n other synonyms that fail me at the moment - the generilisation is this:
Don't talk about it.
It's surprising how much guys think they have to talk about everything. yeah, maybe you could mention it in passing some time -
"-blank-, I just want to know how far YOU feel our physical relationship should go at this time,"
(You could throw a few "Wurd"s, "Dude"s, "sweetie-pie", "honey-darlin"s or whatever in there. I don't know how you talk to your girls...)
But, the best way is to test.
You MUST be very careful. Again, I don't know how far you've gone already, but maybe when you're making out ("Rounding first base", if you would,), just pull her a little closer than usual, let your hand stray casually and gently to the, erm, warmer parts of her body. ((Ass, proally.))
Then, observe for any reaction. If it's bad, stop. If it's good, stop.
Take it slow. If she wants it, she'll go for it - you just have to create the situation.
Anyways, it's late Gotta go beddy-byes.. I'll check in, maybe you can fill me in a bit more about your current relationship. Unless you don't actually have one, and are just asking for "Future reference."