Here are some ways to tell if cats are fighting or just playing:

Observe the cat's vocalizations: If the cat makes a low growl or howl, it may be fighting; if the cat makes a meowing sound, it may be playing.
Observe the cat's ears: If the cat's ears are turned backward and standing upright, it may be fighting; if the cat's ears are standing normally, it may be playing.
Observe the cat's body posture: If the cat backs up and arches its back while its fur stands on end, it is ready to fight; if the cat's body is normally bent and its fur is not standing on end, it may be playing.
It's important to note that the methods above are only a reference for determining whether cats are fighting or playing. The most accurate method is to observe the cat's behavior and make your own judgment. If you are unsure, you can seek help from a professional.