Essentially, evaluating a source means passing judgment on it, not only in terms of whether you agree or disagree with the point that the author is making, but in deciding what kind of source you are dealing with. It's important to use the appropriate kind of source in certain situations. For example, if your professor requires that you use only scholarly journal articles as sources for an assignment, you need to know how to identify that kind of source. Identifying the kind of source may help you in deciding how to judge its content as well.
One of the first things that you should look for in any source material is the author. Anonymous sources should be considered suspect until you have verified the information they contain. Many journals, magazines, and books will not only include the author's name, but give a brief description of who the author is and what his/her credentials are for writing on the subject. If you need more information about the author, you might consult a directory (there are directories for specific professions) or do a web serach for information about the author. If you need help finding information about an author, consult a Reference Librarian.
It's always important to know where an author got his/her information. An author should cite the sources for particular information in the text or list references used in a bibliography at the end of the work. Factual information used to strengthen the author's argument that is given without a source should be verified before you use it to strengthen your own argument. If you need help in tracking down information or a source, consult a Reference Librarian.
You should also notice where the information has been published or who published it. In the case of a book, check the press that published it. A scholarly or university press differs from a commercial press in its goals and stringency. Self-published work, whether in print or on the web, should be examined carefully. If you are looking at an article, notice if it has been published in a scholarly journal or a popular magazine. You can usually tell by looking at the publication. The differences between the two kinds of publication are important. Scholarly journals are refereed, which means that all articles published by them must be approved by a group of known scholars in the field (called "referees"). Popular magazines sometimes report research, but their coverage is at one remove from the actual research, so the description of the research may be very general or may even misinterpret the work. Try to find the actual research if you can.
Always check the date of publication. New research often disproves old.Always be aware of any bias on the part of the author. Bias alone does not disprove a thesis, but it may cause the author to ignore problems in his/her argument. Bias can also result in a misinterpretation or a narrow interpretation of the facts. There are also authors who deliberately misrepresent facts and ideas.