.edu sites: Many .edu sites upload students' papers. .Edu doesn't necessarily mean it's good for academic research. Can the person be considered an expert on the subject?
.org sites: .org sites often have a political, social, economic, or cultural agenda. This means they are biased, which is not necessarily a bad thing as long as you are aware of the bias, you look at opposing viewpoints, and you avoid ridiculously biased, close-minded people. Look for an "About us" page to learn about the organization's purpose.
Authors: Can they be considered an expert in the field? Why? Do they have a reputation for credible information?
Tone: Avoid sources with an emotional tone or overt bias (such as Fox News or MSNBC news).
Purpose: Does the source just provide information? Is the author trying to be persuasive? Are there commercial interests? Is it politically, religiously, or socially biased?
Publisher: Does the publisher have a good reputation for credible information? You can often read about a publication's reputation on Wikipedia.
Editorial process: Is somebody checking the information for accuracy?
References: Does the author cite sources? It doesn't have to be a bibliography. S/he can cite them in the text.