Sure I can tell when I first slide in that pussy. If its tight and there is a expression on her face when I'm going in, then the session will be good and sometimes short. 
 
I do know when the fucking won't be any good also. I'll slide it in and have to hit the sides of her pussy to feel anything or add a couple of fingers to take up the space.

 Then she says, 
"Are You In There Yet?" and I say 
"Damn Your Lose And You Could Drive A Mac Truck Thru There!" Needless to say the session ends pretty quickly after that.