Playing or Dominance

Cooper is a 15 month old golden and our first dog ever. My husband and I never had dogs growing up either. I am having a hard time distinguishing between Cooper just playing with other dogs at the dog park and Cooper being dominate. HOw can I tell when it's time to distract him and get him moving forward again away from the other dog? There have been a few interactions between Cooper and another dog where I felt like it was a fight but am not really sure. There was a lot of growling and mouthing of each other. Is that fighting or playing?