It is only polemics if you take a contrarian view to stir up debate that you are interested in (even if you are taking a devil's advocate position) in order to learn from it and understand people's viewpoints.
Trolls will take a controversial view in order to stir up argument for trivial (or highly emotional) issues such as nvidia vs. amd, intel vs. amd, microsoft vs. apple, abortion vs. non-abortion, atheism vs. religion. The purpose differs. For example, posting a thread on 'is there a god' in SC may constitute a polemics-posting technique, doing it on GD would be trolling. See what I mean?
So you have to consider the intent of the poster, and obviously the previous history of said poster to ascertain whether they are trolling or not?
That could be quite hard to do in some cases, especially with serious topics such as the Abortion and Religion ones you mentioned.