it may occur before you two hit second base, due to your areas of interest. The butt and breast part at least; if you do get that far, and he starts "feeling his way around" tell him to be careful with your breasts, because they're ticklish. If you two have developed playful chemistry, he'll naturally try to tickle them. Lol
I'm scheming up a solution for the belly, but if he starts groping at your posterior, then I imagine a similar taunt will do the trick.
At least, these are discreet. If things start getting romantic, or remotely serious, I think your best bet is to start subtly hinting at your interest.