It should be obvious mostly, butting heads w/ others, chasing, but one that a new to reefing person might not realize when they see it is backing up to fish it does not like.
That would be trying to tail swipe w/ it's spike on each side of tail.
This is why they are also called a surgeonfish, that spike is like a scalpel.
I've been hit once, you'll never forget it!
If you see any fish w/ huge gashes, he would be first suspect, hopefully not fatal wounds, but they very easily could be.
