Let's play "Spot the Cognitive Infiltrator"
I've been on that hunt for a few years now and I've learned a few things.
First of all they often write from a script and a search of suspicious phrases often shows the exact same phrases posted all over the internet on that particular subject.
They don't need to be recent sign ups at FR. I noticed about a half dozen who have been at FR for years but are silent outside of primary season politics. They claim to love conservative candidates but are greatly "concerned" about things like Michele Bachmann's headaches or Herman Cain's rumored marital infidelity. They post dozens of threads about the things that concern them and when all other candidates are destroyed, they grudgingly give in immediately and announce that we should all get behind the winner no matter how bad he is and then they disappear for another 2 to 4 years.
Single issue posters who go against the flow but only for our own good. This is a tricky one because we have legitimate disagreements among ourselves but combine a little phrase searching with a look at their posting history and it narrows it down. As an example see who can spot the troll on this thread (and virtually every other thread about the national popular vote)
Spot the NPV troll
Avoid witch hunting. Better a good clean troll kill than friendly fire accidents.