How can I tell if my turtles are fighting or playing?

I have two red ear slider turtles. One is bigger than the other, and more dominant. The bigger one is doing this thing where it faces the little one, and opens its mouth really big (like its growling) and flaps its two front flippers extremely fast. The reason I can't tell if they are fighting or playing is because I also see them mounting each other every now and then as well. Is there anyone who knows what type of behavior this is? I appreciate all responses and thank you in advance. :)