system
Given the following post, respond as if you're commenting on Reddit:
human
How do felines know when they are fighting or just playing around?

Felines normally play with their siblings or "friends" by scratching and biting, how do they know they are not in danger and how do they know they don't need to fight for their lives?
gpt
Veterinarian here. Playing by biting and scratching is something that is learned at an early age, typically before and around 3 months of age. They learned what is called an "inhibited" bite as kittens. 

The proposed process is that if another kitten bites too hard, the bitten cat will cry out in pain and stop playing. This bums out the biting cat as he/she no longer has a playmate due to the bite, so the biting cat will learn to bite less firmly, and vice versa for the other cat. 

Kittens who are orphaned or not socialized will NOT have this inhibited play behavior and commonly will bite too hard or scratch too much, which is why socialization is one of the most important aspects of raising pets. 

Basically both cats learn how to play with each other over time and learn their limits. 

Another perspective is that dogs and cats are born with play instincts as practice for real hunting. When dogs play catch or tug of war, they are actually practicing for what they are bred for - retrievers chase and bring back balls because they were bred to retrieve birds, terriers will play and rip up toys because they were bred to hunt small rodents. 


To specifically answer your question, how do they know when it is play time and when it is fight time is based on the autonomic nervous system, and how comfortable or fearful the pet is at the time. It's somewhat complicated but if you've heard of "fight or flight" knowing when they are safe is dependent on the context of the environment and situation. If a cat is in a new environment and feeling anxious or scared, most likely there will not be play behaviors, if they are home, their autonomic nervous system down regulates their fear and their play behavior comes out. If another cat is biting too hard and not doing an inhibited bite, they will respond in kind. 

tl;dr - The difference between playing and fighting is contextual and to an extent something learned.