When dogs play, they will commonly roll to their back and display submission, take quick nips at each other, yip and even growl. They will keep their butt in the air and hop around on their front paws and take off running to provoke the other to chase it. These are all practice moves, things they would do if they were wild to take down prey.
You and your dogs will definitely know the difference, dog fights are frightening. If you ever have a dog fight, it is best to remain calm so you can defuse the situation as quickly as possible.
However, you can take preventative measures if you think your dogs might be getting aggressive watch for signs. Either direct eye contact that they refuse to break or refusing to make eye contact. Are the teeth exposed when they growl? What is the situation that leads up to the vocalization? What are the sexes of your dogs? What breeds are your dogs? How old are they?
A lot of times, aggression will come if it is male/male, female/female. They may become aggressive over food or treats.
If you are unsure and you don't quite trust your dogs, don't let them alone together.
"You know, I've given the matter some thought, and I think I'd be willing to be a house pet to a race of super intelligent aliens."