You'd think that the longer you'd been on the Internet, the easier it would be to spot a troll. The problem I run into is that a lot of clueless people sound like trolls. When I talk with people in the real world (tm) it's easier to tell when they're being genuine.
So, generally speaking a "win" for a troll is that they've successfully wasted time. Concern trolls have something quite different in mind but generally speaking if the troll can get you off topic and fighting with them instead of addressing the topic at hand, they've won.
As my time has been pretty limited recently, when I get in conversations where someone immediately goes off topic and doesn't understand what was said initially I recognize it as a troll. The irony here is that as I see the trolling type behavior I get irritated and that starts making me sound like a troll (especially if the initial troll was actually being genuine). Anyhow, recognizing this in myself I've decided just to let the trolls spew their nonsense and give them the down arrow (where available). It's unfortunate though because amidst the troll speak there are usually people trying to get somewhere. I actually like how reddit allows for different comment threads to take a life of their own in that regard (not that it hinders real trolls much). The tools have been making this sort of thing better but I think we still have a long way to go before the web is a nice space for conversation. Human nature being what it is, I'm not sure we'll get there.
Incidentally, it looks like Bush has managed to outlaw anonymous trolling (free speech be damned). I wonder if anyone will manage to overturn that once he leaves office or if we'll just be left with another bit of nonsense legislation. (Yes, it's good to outlaw online harassment, and no, this isn't so narrowly defined.)
Interesting psychological explanation for comment trolling ("deindividuation") on techcrunch.