Q: How can I tell if my girlfriend is faking it when we have sex?
Well, the short answer is that you can't. A woman who knows how her body responds when she's really having an orgasm should be able to do a good enough job of faking it to fool almost anyone.
Now, if you're really worried about it, the best thing you can do is not become the world's foremost orgasm investigator, hoping your amazing powers of perception prevent any possible deception on the part of your sex partners. For one thing, you'll be so busy trying to "read" her reactions, your performance will probably suffer as a result … making it even MORE likely that she resorts to faked ecstasy. For another, your enjoyment will suffer, because you'll be so paranoid about whether she's enjoying herself, you won't be able to relax and enjoy yourself either.
Some women may have an orgasm during foreplay, but not during intercourse. Some women may have orgasms during intercourse, but only in certain positions. Some women are quieter, some noisy. Some women may have smaller orgasms really easily, but rarely have a really earth-shaking one … some women may have multiple orgasms while they're in bed, some may rarely have one at all. And as if all that isn't confusing enough, even when a woman does have an orgasm, it won't always be the same as the last one.
The best thing you can do is either talk to her openly about it, or (assuming that would be awkward for you, and something you'd really rather not do), just concentrate on learning what she likes in bed, and then make sure you do more of it. Do you notice her reacting differently when you do different things during foreplay, and can you tell which things she likes best?
Pay attention to how she reacts to what you do … not with the intent of figuring out if she's faking it, but with the intent of figuring out what she really likes best. You may never find out whether she fakes orgasms or not, but you'll definitely become a better lover, and the more she enjoys herself the less likely she'd ever need or want to fake it to begin with!