FINISHED?

I have a simple question that wont let me relax
How can you determine as a matter of fact?
Whether they're on their fronts or their backs
When an eighteen thirty girl reaches climax?
If really difficult to tell when she never reacts
Some say, "who cares" and just make tracks
Does she drops her nail-file or ask for a snack
Or does the bloke behind slap you on the back