Body language and vocalization (calling out) are vital to determine whether your cat is in play or fight mode. Play is likely to be silent or involve quiet meows and hisses in kittens, and is relatively gentle despite chasing and tumbling.
The cats should not be showing any signs of distress. When this progresses to fighting, cats might show aggressive posturing beforehand with raised hackles, ears back, hissing, and growling.
Cats tend to be very vocal when fighting, and their body language is more aggressive, intending harm to one another. Remember, it can be dangerous to intervene and stop a cat fight. You must be extremely careful that you don't get bitten yourself!