How can you spot a troll?

Let’s say this particular troll does not always take a recognizable name like Randy or always say, “I only want nice people to answer this question.” What other signs are recognizable early on. How can you quickly discern that you’re dealing with a troll, and there’s no point wasting your time trying to placate them or reason with them, because trolls are immune to any such effort.

Oh, and feel free to answer this question even if you are not particularly nice today.