Usually when I find someone posting something I believe is completely false online, I double-check my own belief with reputable sources to make sure I'm not mistaken or out of date. In the 80-90% of cases where I don't find a good argument contrary to my prior understanding, I check the post history of the user in question to see if they're some kind of subject matter expert that might have a deeper knowledge of the topic than the journalists writing articles for a general audience.

Unfortunately that's rarely the case. More often three of their last four comments contain racial slurs and misspellings of two-letter words, which doesn't instill a huge amount of confidence that they know what they're talking about.