User blog comment:Obi the LEGO Fan/New personal attack policy/@comment-4910132-20140127171936/@comment-4845243-20140127185058

It's pretty easy to tell, in my experience at least. You can consider a few factors: 1) are the users friends, 2) was there a fight or some reason for them to insult each other, 3) does the user in question have a record of making personal attacks, 4) does the other user respond by acting offended or hurt, or do they take it as a joke, etc.

And as a last resort, you can just ask the users involved. :P