When a person gets online in this day and age, it can be very tempting to believe that all of the information that you see on the Internet (and there sure is a lot of it) is of sound value and comes from reputable sources. Unfortunately, this is just not the case. However, it would probably not be in your best interests to assume that all of the information you find online is of dubious merit, at least until you have seen a particular piece of questionable info refuted elsewhere. Sometimes it can be easy to determine whether something is not based on solid underlying research, and thus should be avoided. But in other cases, it can come down to a more rigorous process than a lot of online researchers are willing to subject themselves to.
Having a solid degree of research underlying a claim can often take several sources to either prove or disprove. For starters, while the information itself might be counterintuitive at first (such as if it talks about quantum theory), if a large enough number of experts agree to something, it is generally sound. In some cases, especially when the underlying theory is extremely complex, a little bit of educated deference is all right to practice.
Of course, in some cases, an idea is just outright wrong, and it needs to be shown why the idea is wrong. For instance, say that someone wrote online that slavery was not only an injustice against an entire race of people, but was ultimately economically unfavorable. While the injustice part is unquestionable, the economic reality was demonstrably far different than such a claim would posit. When you check out any kind of source online, you have to evaluate why they would say what they say- remember that there are trolls everywhere, working to hurt people for their own amusement. The trolls will lie constantly.