Not all of the material you uncover during your research process with be useful for your particular project. It is important to evaluate the credibility of any source before you rely on the information for your own projects. When evaluating print materials, you will first want to determine what type of publication is it - is it a scholarly or popular type of publication?
Second, you should determine the bias of the publication. Figure out for whom the article was intended and determine if it is a balanced treatment. If it is not a balanced source, does it support the angle you wish to pursue? It is also a good idea to check the credibility and reputation of the author and the publisher.
Finally, you will want to make sure that your information is relevant to your research, particularly is you are examining an older source. The University of North Carolina at Pembroke provides some thought-provoking questions that will help you evaluate your sources (archived web link). Web pages in particular can be tricky to evaluate since anyone can publish anything they want on the Web. The UC Berkeley Library has a superb tutorial including important questions to ask yourself when evaluating web pages. Click here for more tips on evaluating sources.
Some other useful sites include:
This web site was developed by the librarians at Cornell University and is a good starting place to learn about the types of questions you should be asking concerning the appropriateness of resources you may find while conducting research in the literature.
Developed by the librarians at Widener University this web page highlights those features that you want to look out for in a web page that will help you determine whether or not it is a good site.
An excellent web site developed at Johns Hopkins University.