One way of telling if they are real is if the woman gets flushed, not just in the face but in the chest. I don't know how that can be faked.
An interesting paid site I visited is a preview site for an ****** agency in Phoenix, where they have some of their escorts in action.
Premium Link
Upgrade
Can't remember the name unfortunately, but one of them is definitely fingered to a nonfaked orgasm, she's flushed and shuddering, bites her lip to keep from shrieking too loudly. Another one called Natasha doesn't cum, but she fingers herself to the edge, you can see her body tense up and once again the flush covers her.
I found that this particular ****** advertising site had a handful of the best porn vids I have ever seen. The girls are pros in one sense, but not pros in another, so there is less overt acting. As a matter of fact, I talked to one of the girls, they weren't even paid for the videos -- they did it as advertising, plus they thought it would be fun.