How do you do a fact check?

On YouTube videos or Google/YAHOO News, Wikipedia, ect.

The internet and friends keeps telling me to do fact checks, but I don’t know how to do it. How do I do them?

For starters how do I do a fact check on YouTube videos on IBS and low FODMAPS foods. I get conflicting information on foods that are good to eat with Irritable bowel syndrome.

Humor and legit answers welcome.