How to tell what is 'widely accepted' or 'consensus'
Firstly, the "official" ways that researchers state their opinion is by stating it in either a publication or in a presentation at a conference. It is from here that we can gain any idea of what any researcher thinks about a given topic.1

That being said, the only way to know whether something is "widely accepted" is to be familiar with most of the current (academic) literature on a given topic. Only by really having read the publications of most of the preeminent researchers in a given field can you really get a sense of what the consensus is on that topic. This, of course, requires a lot of time and expertise (and, if you don't have access to a university library with journal subscriptions, money). It took me years as a graduate student to learn what was the consensus about certain topics within my own field.

Because of this, anyone can state anything and most people won't know the difference. The easiest way to really determine if it's true is to ask two or three recognized researchers if they agree with the statement of interest. You can do this via email, but they likely won't get back to you. You can ask grad students in their lab and the answer will be that much less reliable, but still close to the source. Beyond that, you'll just have to find experts you can trust and rely on them.



1 Obviously, researchers are people like anyone else, and will discuss their pet theories with friends and colleagues over email and in person; it's very hard to quantify these, and you only rarely see this sort of discourse being formally recognized. Occasionally a publication will cite "unpublished discourse", but that's a rare occasion.