What is the Difference Between Cats Play Fighting and Real Fighting?

We have two feral rescue cats who came into our house about a year-and-a-half apart. They have their separate spaces in terms of perches, litter boxes and feeding stations.

They tend to keep to themselves quite a bit. Sometimes they do fight and I wonder whether it's just play or aggression. There's plenty of hair on the carpet in their aftermath, but no one's injured and they don't hiss at each other.

I caught them last night (that's Cali on the left and Chloe on the right) in a rare moment of togetherness.

Cats Play Fighting