How do I know if my dogs are fighting or playing?

I have two dogs, a German Shepherd, and a Black Labrador. The GSD is around 9 weeks old and the black lab is around 7 months old. I recently introduced the GSD to Alexa (the lab) a week ago (she was a christmas gift) and they seem to be playing (or fighting) a lot. They get on the floor and start wrestling all the time. Whenever I hear any growls or squeals I separate them. It can get pretty intense sometimes. They are both females. Is there any way to know when they are fighting and not playing?