Is the author well known and frequently published?
Are there any reviews available for the author's work?
How easy is it to find contact information for the author?

Is the publisher well known and well respected in the industry? Do they have a website?
How selective is the publisher in determining what they publish?
Are they also the main retailer for what they publish?
Does their other content seem legitimate and credible?

When was the source originally published? When was it last updated?
Are you citing the latest version? If not, how does that affect your argument?

Does the argument the author makes appear anywhere else? Is it backed up with data or other sources?
Where does the information presented come from?
Are there grammatical or spelling errors? Any dead links?

What is the tone of voice? Does it appeal more to an academic reader, or is it more casual?
Would someone not familiar with the subject be able to understand the source's topic after reading it?
What is the intended audience of the source? Is it similar or different from your intended audience?

If the source is a website, does the site have ads? Do they affect the content?
What is the purpose of the source? Is it to persuade or argue? To entertain or inform?
What is the author's tone of voice? Do they seem to only present one side of the argument? How do they address the counter argument, if

Does the author give credit to sources where they received information and conducted research?
Do the sources they used seem legitimate?
What sources refer to the one you are using? Do those seem legitimate?

Is this the original source? Has it been reproduced?
If it was reproduced, when was that done? Who reproduced it? To what end?
If it was reproduced, does it have copyright information or information on the original source?

How much information can be learned from the source on a particular topic?
Does it talk about a broad topic, or a specific element of a topic?
Are there larger, more popular sources on the topic than this one?

After reading through the various questions above, does this source seem credible?