Who wrote the source? Can you find an author or authors listed on the source?
What kind of background does (do) the author(s) have? Try Googling their name -- what else have they written?
Who is the publisher of the source? What interests might they have that would shape the way the source is written?
What
What is the source about? Does the author have the appropriate knowledge or experience to write about the topic? What are their credentials and affiliations?
What information is the author citing? Do they provide references? Can you track down their sources?
When
When was the source written? How might the culture of that time influence the content?
Can you find a date anywhere on the website or source?
Where
For a website, what is the URL?
Sometimes you can tell more about a website based on the end of its address (e.g. .edu, .com, .gov), but this is not always an indicator of reliable information.
Where was the source published, if not online? What perspective is it coming from?
Why
Think back to the author of the source, if you found one. What possible motivation could they have for writing the source?
Does the source have a clear message? Does that message align with the author's or website's intentions?
How
How did you find the source? Through the library, the open web, or somewhere else?
How is the source getting its message across? Is it using particularly persuasive language? Why?