Although not a form of written evaluation in and of itself, source evaluation is a process that is involved in many other types of academic writing, like argument, investigative and scientific writing, and research papers. When we conduct research, we quickly learn that not every source is a good source and that we need to be selective about the quality of the evidence we transplant into our own writing.
When you conduct research, you naturally look for sources that are relevant to your topic. However, writers also often fall prey to the tendency to accept sources that are just relevant enough. For example, if you were writing an essay on Internet censorship, you might find that your research yielded quite a few sources on music censorship, art censorship, or censorship in general. Though these sources could possibly be marginally useful in an essay on Internet censorship, you will probably want to find more directly relevant sources to serve a more central role in your essay.
Another point to consider is that even though you want sources relevant to your topic, you might not necessarily want an exclusive collection of sources which agree with your own perspective on that topic. For example, if you are writing an essay on Internet censorship from an anti-censorship perspective, you will want to include in your research sources which also address the pro-censorship side. In this way, your essay will be able to fully address perspectives other than (and sometimes in opposition to) your own.
One of the questions you want to ask yourself when you consider using a source is "How credible will my audience consider this source to be?" You will want to ask this question not only of the source itself (the book, journal, magazine, newspaper, home page, etc.) but also of the author. To use an extreme example, for most academic writing assignments you would probably want to steer clear of using a source like the National Enquirer or like your eight year old brother, even though we could imagine certain writing situations in which such sources would be entirely appropriate. The key to determining the credibility of a source/author is to decide not only whether you think the source is reliable, but also whether your audience will find it so, given the purpose of your writing.
Unless you are doing research with an historical emphasis, you will generally want to choose sources which have been published recently. Sometimes research and statistics maintain their authority for a very long time, but the more common trend in most fields is that the more recent a study is, the more comprehensive and accurate it is.
When sorting through research, it is best to select sources that are readable and accessible both for you and for your intended audience. If a piece of writing is laden with incomprehensible jargon and incoherent structure or style, you will want to think twice about directing it toward an audience unfamiliar with that type of jargon, structure, or style. In short, it is a good rule of thumb to avoid using any source which you yourself do not understand and are not able to interpret for your audience.
When choosing sources, consider the quality of writing in the texts themselves. It is possible to paraphrase from sources that are sloppily written, but quoting from such a source would serve only to diminish your own credibility in the eyes of your audience.
Few are sources are truly objective or unbiased. Trying to eliminate bias from your sources will be nearly impossible, but all writers can try to understand and recognize the biases of their sources. For instance, if you were doing a comparative study of 1/2-ton pickup trucks on the market, you might consult the Ford home page. However, you would also need to be aware that this source would have some very definite biases. Likewise, it would not be unreasonable to use an article from Catholic World in an anti-abortion argument, but you would want to understand how your audience would be likely to view that source. Although there is no fail-proof way to determine the bias of a particular journal or newspaper, you can normally sleuth this out by looking at the language in the article itself or in the surrounding articles.
In evaluating a source, you will need to examine the sources that it in turn uses. Looking at the research used by the author of your source, what biases can you recognize? What are the quantity and quality of evidence and statistics included? How reliable and readable do the excerpts cited seem to be?