SEE HERE It isn't at all hard to tell when you're dealing with trolls and this article sums it up pretty concisely with the following points:
1. Making outrageous comments designed to distract or frustrate.
2. Posing as a conservative and making comments that discredit the movement.
3. Dominating discussions.
4. Posting prewritten responses.
5. Making false associations.
6. Exhibiting false moderation.
7. Exhibiting false moderation.
Read the post for development and further points. It's helpful when you're trying to decide whether you're just dealing with an idiot or a troll. It's often hard to tell the difference.