You know when she's faking it, you just don't want to admit it to yourself. Maybe she moans too loud, or not loud enough, and she doesn't go slightly cross-eyed like she usually does. Life is not like that classic scene in When Harry Met Sally—when your girl gives you her best Herbal Essences commercial audition, you know deep down that you didn't really get it done.
It's OK if you don't get her off every time (as long as you've made an effort). But if it becomes a habit, she might upgrade to someone who can.