There's a lot of partisan misinformation on the Internet. This isn't a new problem. Orwell wrote in 1945:
Much of the propagandist writing of our time amounts to plain forgery. Material facts are suppressed, dates altered, quotations removed from their context and doctored so as to change their meaning. Events which it is felt ought not to have happened are left unmentioned and ultimately denied.
When I'm following an argument and trying to figure out who's right, I usually try to do some "triangulation" to check the reliability of the sources that are being cited. To check what author A says about X -- assuming that X isn't a subject I know much about -- I find a number of techniques to be useful:
Find something I do know about, and see what A has to say about it. Same principle as checking the reliability of a telephone book by looking up your own listing.
Find out what other people have to say about X.
Find out what other people have to say about A's discussion of X -- book reviews, for example.
The Internet and Google make it pretty easy to do this even for an author you've never heard of. Of course, once you find someone commenting on A, you then need to check their reliability, and so on. I look for longer articles or essays (or even books), where the writer has the time and space to present their reasoning at length.
Wikipedia is better than nothing--it's very convenient, and probably not bad for basic facts, through the work of editors who keep an eye out for vandalism --but Encyclopaedia Britannica is more authoritative and coherent, and not very expensive (it's currently available as an iOS app for $15/year).
I find the New York Review of Books archive particularly useful, especially on history and politics. An online subscription (currently $69/year) includes access to the full archive of more than 15,000 articles, dating back to 1963. The name is a bit misleading: it's not like the book review section in a newspaper, like the Globe and Mail's, where the reviewers are journalists who laud noteworthy books. The NYRB's reviewers and essayists are brilliant writers who are often experts in the field. And they don't hesitate to express harsh judgments and get into heated arguments. (My favorite barb is from Hans Morgenthau's 1964 review of View from the Seventh Floor, by W. W. Rostow, chair of the US State Department's Policy Planning Council: "Mr. Rostow has a powerful, brilliant, and creative mind. How could such a mind produce such trash?")
For academic topics, I check sites like Google Scholar and JSTOR (which now provides limited free access for individuals). These days, many academics also have blogs or websites where they publish a lot of their writings. Two good examples are Paul Krugman (economics) and Joseph Heath (political philosophy), who write both for other academics and for a popular audience.