Ask Lemmy

• Who is publishing it? How trustworthy are they? What's their track record? What's their funding and goals?
• What's the source? Can I verify the source? Do I need to or to what degree does it fit other information I sighted or assessed previously?
• Who is supporting the information? Experts of the field? Of long investment? Or far-fetched or arbitrary people that may not have any expertise, may not have fundamentally verified their own information and biases, or are not trustworthy for other reasons?
• How recent is the information? Is there even legitimacy or urgency in giving it attention right now? Is there on-site documentation? Official analysis reports or scientific studies? Of what quality, by whom, with what on-site expertise?

Something like that.

For stuff I'm not sure of or unknowing I often check Wikipedia, which links further sources, or actually check court rulings, or laws, or state published information, etc. Having watched many documentaries and having read articles gives some assessment basis for various related topics. Watching or reading from independent invested journalists, especially when they go/went on-site, gives (reasonably) verifiable legitimacy.

As for your friend, it also depends on how much you are willing to invest. Nudging with questions like "don't you feel it's suspiciously positive?" or dropping some information like "he was bankrupt multiple times, why is he celebrated as saving the economy? doesn't that seem off?"

If they're open to other sources and information, it'll be easy. More likely, they are not, which will make it harder and a longer process if you're willing to invest. Factual information often doesn't help. Making them question stuff themselves would be the best way then.