This has probably been asked a hundred times, but anyway.
How to tell the difference between play fighting and real aggression?
Dante has a curly tail that never unfurls except when he's asleep (and so is useless for gauging his thoughts with), but when he's been at the catnip and Evey slinks into view, his fur stands on end, his ears pin back, and he pounces on her. Evey's ears don't pin back nor fur stand on end when she wrestles with him, but she flicks her tail like a whip while she waits for him to move, and lies close to the ground. Neither of them make any noises during any of it.
I'm guessing this is all play fighting, cos they get along fine when there's no catnip around, but how would I know if they did start going at each other for real?