Forgot to add that in my experience, if the cat doesn't hiss, it's play. If there's a hiss, then someone crossed the line. I used to be confused in the early days of having Buffy (the first time we had more than one cat at once, with both being okay with the other's presence) when I'd swear they were fighting. They'd tussle, then one would run away. Then she would come back, peer around a couch corner, wiggle her hips, and dart at the other cat, and the fight would continue. So I go by the hiss rule, lol. Though, just because one cat hisses doesn't mean the other will quit, and that's when you step in. I remember back when Buffy was still a baby and she and Willow were playing ALL THE TIME (Willow couldn't believe her luck at having a playmate, finally, and Buffy simply had a baby's endless energy). Well, naturally, due to Buffy's endless energy, Willow would eventually tire of play and would want to spend time by herself. Buffy didn't agree with that idea, and once (I didn't see it, my sister replayed it to me), the two were racing throughout the house, a common game for the two. Only, this time, as Willow blasted past with Buffy bringing up the rear, my sister swore she heard Willow hissing with despair. But Buffy continued to bring up the chase, so my sister had to step in and save poor, tired Willow.
Also, if fur flies (literally), that's a fight, not play. Willow looses puffs of hair from her feet when she's distressed, so when she and Buffy are fighting, there'll be clumps of fuzzy grey fluffs on the carpet and on Buffy. Then again, sometimes she'd puff out feet-fluffs when they're playing very intensively. Eh, basically you just need to know your cats' behaviours and temperments to figure out if they're playing or fighting. But whatever they're doing, they can handle themselves. If they can't, you'll know it, and that'll be when you step in (but simply separate them, don't force them together or "spank" them or squirt water. Cats are different than dogs, and all you want to do is remove the circumstances that have upset them. Add in negativity, and they won't respond like a dog would, and it could backfire in your face. Trust me, I speak from experience. Why else does Buffy still hate Molly with a passion, 3 years after Molly's arrival?... lol)
