Mercola is a doctor of osteopathy who has frequently been targeted by the FDA for promoting false, misleading and even dangerous medical advice. His site promotes products and his blog includes false and/or misleading information about medical topics.
When you open up a news article in your browser, open a second, empty tab. Use that second window to look up claims, author credentials, and organizations that you come across in the article.
Fake news spans across all kinds of media - printed and online articles, podcasts, YouTube videos, radio shows, even still images. Be prepared to double-check everything.
Beware of confirmation bias. Just because you might agree with what an article is saying doesn't mean it's true.
As Mad-Eye Moody said in Harry Potter and the Goblet of Fire, "Constant Vigilance!" Always be ready to fact check.
Even the best researchers will be fooled once in a while. If you find yourself fooled by a fake news story, use your experience as a learning tool.