In scholarly articles like this one (Ferrante, et al, "PCB levels in adipose tissues. . .", Chemosphere) , it is easy to find out where the author(s) got their information. You just look at the references!
However, most non-scholarly sources are not going to have an organized list of references at the end of the article. Instead they will have embedded links throughout the article. In the following article (Levine, "The majority-black city blocked from electing black officials," The Guardian US), embedded links in red text lead to (in order): legal case file, US Supreme Court brief, local newspaper article, Census Viewer, and a US Census Report.
Click (or hover over) the links. What types of sources do they lead to? If it is to other credible sources, then you know this author did their research! If there are no links or if the links lead to "red flag" sources (see below), then you should be wary of using it as a source.
Some sources are red flags. Watch out for:
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