How to tell when kitten play gets too rough?

I have two 9 week old kittens, one boy and one girl (siblings) that love to rough house with each other. The boy, Ash, is a little more hearty than his sister, Suby, and loves to pounce on her and chomp down wherever possible. Most of the time she will let out a yelp when he does that, but for the most part tails continue to gleefully wave through the air. My question is, how do I tell when the rough play goes too far and needs to be broken up? It's usually the girl that cries out, and I know from experience that Ash sure does bite real hard......I just want to avoid having the kitties feel negatively towards each other.