text: If you do it right, not very often.  My wife will tell me if she got off, but most of the time the scratches on my back and the muscle spasms will let me know.  I really think the key to it is communication.  That's where intimacy starts.  If you think she isn't getting off, go crazy with the foreplay.  Tease her forever and forget about your immediate needs, make her lust crazed and with a little imagination it shouldn't be much of a problem.  Unfortunately a lot of guys forget that a woman needs the mood to be perfect.  Guys just take friction, for girls it's about the intensity and lol.. how stressed they feel when they're foolin around.  You have to be able to tell what mood they're in before you start.  I switch it up alot.  Rough is fun, but not always right off the bat.  Make it interesting, be creative but communicate with her to make sure that she's ok with what you're interested in trying.  You should know, even if she won't tell you straight out.  They don't want you to feel insecure, that's embarrassing, especially after the sex.  She wants to make you happy, even if she isn't totally getting off, or she wouldn't be fooling around.  Don't bug her about it, what ever you do, sex shouldn't be a chore, it's about exploration of yourself and your love.