Who? Who wrote this? Can you even tell? Are they an authority in this topic? Credentials are important, but first hand accounts are also important. Most importantly: who stands to benefit if you believe this source?
What? What kind of resource is this? Is it an advertisement? Newspaper article? Scholarly research article? Also, what kind of information does it present? Does the content accurately match up with what you already know about this topic? And another thing: are there a bunch of advertisements, either related or unrelated to the topic of the article?
When? How up-to-date is the information? And how soon after an event was this published? (We've all seen false reports and misinformation happen shortly after major events like school shootings.) Also, how up-to-date do you need the information to be? Looking for reviews of classic movies that came out shortly after the cinematic debut versus critical acclaim that came years later can make a big difference.
Where? Country of origin? Where is this information in relation to the structure of the website? Is it on the front page? Is it buried?
Why? What's the purpose of the source? Is it trying to sell you something? Convince you of something? Share facts? Also, why are you looking at this source? Entertainment? Medical research? Academic need?