The simplest way for me (which has been absolutely foolproof) is to talk to her about a friend of yours who won't stop seeing this guy who's just an asshole to her. She'll naturally ask why you think she's with this guy. Reply that she told you it was because he had a big dick.
I guarantee either the conversation floodgates open up wide, or she'll clam up and not talk about sex at all. Either way, you'll know what she's made of.
Sidenote: Sadly, my story about my friend is a true one. Coincidentally, she worked in the same office as me and we had a lot of lunch conversations. She moved on and got a job with another firm. Last I heard she was still with the same asshole.