You don't need to "look up" anything if you're a forum member for long enough to recognize sarcasm. And if you're new, you either wait for the others to reply, or act like a noob and reply having no idea if the post is sarcasm or not, and then are promptly corrected, maybe with an image of Picard facepalming.
Needless to say, not a viable option for a filtering program. Also, this assumes the person in question is any good at internet sarcasm. If they're not, and the claim isn't too extreme, you need to start assuming. My approach to extreme, but ambiguous claims, stupidity and so on, is to always laugh them off, on a basis that if it's a joke, we're meant to laugh, and if it's not, then laughing at the claim is one of the better insults you can use. Especially that doing it the other way (taking everything seriously), the covering behind the flimsy "Poe's law" if you're off mark results in coming off as dour and probably insulting intelligent, but more humorous people as a side effect. Unfortunately, the government will be looking precisely for such extreme claims, so "laugh it off" default behavior isn't a good idea, either.
Now, here comes the hard part: you can tell if someone is sarcastic/joking or not by checking how other people reply. Easy for a human, mind(CPU?)-boggingly difficult for a computer of any kind. You need to catch context of the discussion, the person's feeling on the subject, essentially comprehend the entire conversation. Needless to say, if a computer does that, it'll have the right (and probably capability) to call itself an AI.
