If you see someone respond to a question with a post that is argumentative to others, but does not answer the question posted, you may start thinking of them as a troll. If they do that repeatedly, they will be considered a troll.
A big clue is if they argue one side on a question just to argue, but argue the other side on a similar question, merely to get a rise out of others.
Some trolls post a question that seems to lean towards a position, but the argue vociferously with anyone that agrees with the original slant of the question.
By the way, i am not a mod or anything, I don’t even know if the Fluther powers that be even have a guideline for designating someone as a troll. I am only offering how I decide someone is trolling.