The timeliness of the source.

What is the date of publication? Is there a newer edition?
Science, medicine, and technology demand the most recent information and developments.
History, literature, and other humanities may require older information. 
If you need a historical perspective, then you will obviously use older sources.  
Did your professor set any limits on how old your sources may be?
As a general rule, sources published within the last 5 to 10 years are appropriate.

The uniqueness of the content and its importance for your needs.

Does the source help answer your research question?
What is the depth and breadth of the information presented?
Does the source provide the information you need?
Does it offer sufficient information related to your needs/purpose?
Would you be comfortable using this source for a research paper?
Who is the intended audience - general public, professionals in the field, practitioners,?
Is the information unique? 

The author's credibility and expertise on the topic. The publisher's reputation.

Who is the author? Are they an authority in the topic? What kind of reputation do they have?
Is the author a journalist, researcher, professional, politician?
Can you find any information on the author's credentials, education, or expertise?
Who is the publisher? Is it an academic press or popular press?
Is the author or publisher known to have a certain bias or viewpoint?
Is the author affiliated with an organization or institution? What do you know about the organization or institution?

Reliability, truthfulness, and correctness of the informational content. 

Where does the information come from? (consider the author and publisher)
Are the original sources of information listed? Are there footnotes? Is there a works cited/reference page?
Can you verify any of the information in independent sources (such as a book), or from your own knowledge? 
Based upon your knowledge of the subject, does the information seem accurate?
Has the information been reviewed or refereed by subject professionals?
Does the language or tone seem biased?
Is the information easy to understand? 
Is it well written? Are there spelling, grammar, or other errors?
 

The presence of bias or prejudice.  The reason the source exists.

Is the content free of bias?
Are possible biases clearly stated/easily detected?
For periodicals -- Are the advertising and informational content easy to distinguish?
Is it an editorial?
Why was the source published - to inform, teach, educate, enlighten, sell, persuade?
What is the author's intent or message?
Is the purpose of the page clearly stated?  Does the content achieve its intended purpose?