Are my rats fighting or playing?

I have two female rats, Ollie and Iggy. Ollie is always jumping on top of Iggy and biting her and chasing her around. It is never Iggy that starts it. Ollie also bit me a lot when I first got her. I don't know if they are fighting or playing. They have a huge cage and tons of room so I don't think its territorial. How can I tell?