She's looking at you
This one can be a little tricky, because eye contact is totally hot and can heat things up quickly for women. What can I say? We love feeling connected.
That said, orgasms dilate our pupils. Our brains turn off. We can't focus -- sometimes even for several minutes after the orgasm has passed. So you should be looking for a glazed, faraway look, rapid blinking, rolling eyes... basically anything but a dead-on stare.
Her skin is dry
Orgasms cause faster heart rates, higher blood pressure, and quickened pulses. So even if you're doing it in a snowbank, she is going to be sweating. The perspiration might not be everywhere (damn snow bank!) -- but it will be in the creases of her elbows and knees, between her breasts, and at the small of her back.
You can still see her clit
I'm going to assume you all know where the head of a woman's clitoris can be found for this next tip (if you don't know, please, please, please stop what you're doing and look it up). In the moments leading up to a woman's orgasm, her clitoral head actually retracts up into her folds. And you don't have to really be looking to see this evolutionary wonder; you can feel it. With your finger. Which you should have there anyway, since we love that.