If I see Daisy lip start to curl and I can see her canines than I know shes getting fustrated. Thats when I tell her to play gentle or a seperate for a bit. You can also tell by their body language. Her tail isn't wagging no more, and she has a certain growl that, just her whole body language says, "No more I've had enough." Peaches is learning how to play since she was taken to early, and Daisy will give her a slight nip on the scruff of her neck to let her know shes too hyper and needs to calm down. Rocki, my boxer, likes to play too, and she forgets her size and will slap down with her paws so I have to remind her to play gentle and lay down. Rocki does gum bite, she basically places her jaw on the scruff, and apply light pressure to let them know its calm down time, this is after she has stood up and walked away and they have followed her. The cats will slap when they've had enough and walk away.
As long as no one is yelping, crying, or the sounds that means things are no longer playing, I don't really worry. But I always have my eyes on them just to make sure.