I didn't really realize this was something people had to ask about. Now, when I think of rough sex, I just think of doggy with hair pulling and spanking...no more. And as that as my definition of rough sex, I've always been able to tell without having to ask. Usually, if I read the signs, I bring it up before we've even kissed (but of course, we're in a situation where we've been flirting for a while). Like I'll tell her that I know (don't say "guess," say "know") what her favorite position is. Girls have always loved me guessing that correctly. I get it right because the answer over 80% of the time is doggy (I'm only speaking of my experience). So, it makes me look like I know what I'm doing in bed because I "read her." I just say doggy everytime. So, like I said, I've got like a 4 out 5 chance of being right. You know what happens if you do get it right? Usually she'll open up and tell you what else she likes right then. She'll say "you're right and I like getting my hair pulled and sometimes spanked." I've had that happen like 3 or 4 times.
Also, there's a fundamental problem in asking. The whole psychological high of rough sex is the dominance/submission element. Asking reverses roles. It makes the asker the submissive one and the askee the dominant one. That misses the whole point. So, like I said above, I try to use a technique to get around that. If a girl wants you to "take her hard" then she's probably not begging for you to "be a gentleman and ask if she will let you." See the problem? Confidence in who you are what you want is usually attractive to women.
So, just in my opinion, if you want rough sex you should (in the flirting stage) bring it up. Try the technique I listed above or somehow come up with a way to talk about it beforehand. That way, you avoid the whole 'rape' question people are concerned about and you get the greenlight without asking (which hurts your game). If you can talk about this while you're still just flirting...it's like foreplay...its very hot.
Remember, I'm not suggesting you just do it; I'm suggesting try a better way of finding out than asking.