You can tell right off the bat, but it just needs experience being a troll (to understand a troll, you must be one). Otherwise, you'll just be left in the dark. You could be trolling using this topic as bait. I could be trolling using this post as bait. Just waiting for a casual user to post something idiotic so I can mark the back of my laptop with another kill. Trolls are trolls. The intention is to get people pissed off or worried while gaining attention to these people.

I thinnk most trolls tend to go for easy and uncreative targets. Popular topics (and fans), sensitive topics/people and idiots are probably what I identify as very easy targets. You just need to say something that will either obviously or in a direction that would make people pissed off. Sometimes only a slight push from a person who might not have trollish intentions will set off a shitstorm when it comes to these three. Also people who troll places by spamming NSFW disgusting pics can be placed in this category as well as grief players in multiplayer games (My favorite was getting a hylozoist card and mass produce polymorphing monsters in newbie areas into MVPs and similar on Ragnarok private servers to piss off admins). These trolls are fairly common and comprise of most of the internet trolls. They're assholes.

The special cases are those who start creating hoaxes in order to create shitstorms. It's only a level up, but they put some actual effort rather than being simple pricks. The pranks they pull can be simple, short-lived memes, to elaborate shitstorms causing controversy outside of the net. These trolls are bored geniuses (bonsai kitten) or cold hearted pricks (myspace moms getting random teenagers to commit suicide) depending on how malicious their pranks are. Sometimes these trolls will just start a topic/prank and let the userbase get into it with a minimal, but well planned effort. I'll just call them classholes for the sake of xkcd familiarity.

As for censorship: I think it's fine. If a website decides to start deleting certain statements, then that's fine because websites are either government or private property which both have legitimate reasons as to censor (objectable opinions, sanctity of the website, user base). However, I don't like censorship relating to politics and world interaction by the media (news, information services) for selfish gains as it would be equivalent to lying to people. Censorship is actually a fun tool to troll with, however. Start up a trolling post on a forum with some weird topic ("Men are useless" from personal experience, I am a guy that wanted some cheap laughs), then start deleting posts that are not in favor of the view in the OP.