how can you tell play fighting from real fighting?

[MimiSam]

We have two new rescue cats, one about a year and half and one kitten of 16 weeks. They mostly get along fine, but they do occassionally fight and I am worried the big one will hurt the little one, as he pins her down and looks like he is biting her (though I haven' seen any injuries). She will go back for more after a minute or two, so she doesn't seem overly concerned, but it looks serious to me and I separate them if I can... should I leave them alone?

Oh you'd know. The preparatory noise to real fighting is dreadful and then it's whirligigs with fur flying. (And she's going back to him - so it will be play.)

I'm wondering about it, though. I assume he's neutered but is she? If she's not, at coming up to 4 months, it's just possible that she's starting to come into an early heat and he may be reacting to it even though he's neutered. How is he pinning her down and where is he biting her? Back of the neck or thereabouts?

Agree, there's lots of yowling, hissing and spitting involved in a real fight. The ears go back, fur rises, much circling happens, and yes, fur flies when they engage. If none of that is happening, it's play.