In doing research, whether online or offline, there are two types of assertions you can encounter: supported and unsupported. (Just like here on Stack Exchange!)
An unsupported claim isn't worth very much. Some blog post says "X", but somewhere out there is another blog post saying "not X". This happens at sites that look more credible than blogs, too. And some of them might sound credible, until you realize that the author has a vested interest or the post is 7 years old and things have changed.
This is why Wikipedia, for example, bars original research and demands citations. When you come across a (well-written) Wikipedia page, or some other source like it, you'll see a bunch of citations at the bottom. Those citations explain where they got their information. (For more about using Wikipedia, see this question.)
Don't just say "there are citations; done!", though. Citations can be misunderstood or misrepresented, particularly in works that aren't carefully reviewed. You have more work ahead of you.
If you need to verify a claim, find its citation and then go look at that source. If it has what you need, great. If not and it has citations, follow those. Iterate until you find what you need or you reach a dead end on all threads.
When evaluating a source, look for these key factors, taken from this quick guide to evaluating a source's credibility from UC Berkeley:
- Authority - Who is the author? What is their point of view?
- Purpose - Why was the source created? Who is the intended audience?
- Publication & format - Where was it published? In what medium?
- Relevance - How is it relevant to your research? What is its scope?
- Date of publication - When was it written? Has it been updated?
- Documentation - Did they cite their sources? Who did they cite?
In this answer I've talked mostly about #6, but everything on that list is important.
Investigating sources is a standard tool of academic researchers, reference librarians, and (good) journalists, among others. If you don't have access to an academic researcher or a journalist to learn the technique from, make friends with the reference librarian at your local library. That person will be able to help you decipher references, hunt down obscure sources, and so on.
You might find these resources collected by Tulsa Community College about evaluating sources helpful. (Many universities have such guides for their students; that's just the first one I found.)