In order to avoid any embarrassment I normally keep a close eye on the time and leave when it is up.
However, one lady I saw set the timer on her phone... so you can't argue with that...
The other clue, on a longer booking of 1 - 2 hours is when you can tell by their body language that they've lost interest, or in one case when a lady said, 'I just want to go home to bed!!!'