It’s usually very obvious from the content of their posts.
For example, in this thread in which the OP (the post) is a pile of blithering nonsense, and the OP (the poster) has been suspended for a month for general idiocy, we find the following posts:
#56
rboyce understands atmospheric science better than most. He’s right that climate change probably isn’t human’s most existential threat.
#96
Nice Wikipedia search. Unfortunately you don’t understand any of it.
That last one was purposeless gratuitous hostility in response to my posting of facts. Granted, you then apologized in an apparent effort to regain some plausible deniability, and then went right on trolling.
tl;dr: What I’m saying is, look in the mirror, troll.