A pretty telltale sign is when she has to ask you to stop because she's too sensitive. Ordinarily, my girlfriend likes me to keep my hand there (or whatever part of my body happens to be there at the time), but she doesn't like me to move around much any more. She likes the pressure.
If, on the other hand, I stop before she orgasms, she gets this really funny look on her face like "WHY THE FUCK ARE YOU STOPPING?" Sometimes I do it just for the face
You can usually tell though. There is ordinarily a point where it shifts from body rocking, screaming, shivering to slow, even panting and soft touching.