How recent was the information published? Is the information outdated for the topic you are studying?
Who is the author? What is his or her experience with this discipline or topic? Is he or she qualified to speak on this area of research?
Everyone is biased, and sometimes authors of information do this intentionally to persuade you. SourceWatch is a tool that can help you evaluate sources and recognize when an item is "sponsored" by an individual or organization that is attempting to persuade readers, rather than inform.
This can tie in with authorship and sponsorship. Ask yourself, what is the intent of this source? How will this affect my research?
Is the information found on this source (and, perhaps, the sources it cites within) accurate? If an author lists the wrong dates for World War I, you may want to question the accuracy of the rest of the source's information.
Hacker, Diana, and Barbara Fister. (2009). "Tips for Evaluating Sources." Bedford/St. Martin's.com. Bedford St. Martin's, n.d. Web. 19 Aug. 2011. Retrieved from http://www.dianahacker.com/resdoc/tips.html.
Quaratiello, Arlene, and Jane Devine. The College Student's Research Companion: Finding, Evaluating, and Citing the Resources You Need to Succeed. New York: Neal-Schuman, 2011.