Basically impossible to tell.. All fish, and especially so in marine fish (IMO), can have their own personalities. I've had clowns that are downright brutal to every other fish in the tank, and then I've had other clowns that are perfectly peaceful. Same goes for most of the things I've kept, especially crabs.

For the most part, as long as you have decently tempered fish (i.e. No assholes) and you don't stick a fish in with another fish it considers food, your good. Mainly stick away from damsels. Damsels are dicks, that you can never get out of your tank.