1) Accuracy:
Verify the information you already know against the information found in the source.
2) Authority:
Make sure the source is written by a trustworthy author and/or institution.
3) Currency:
Depending on your subject, your currency needs will vary. For topics related to recent breakthroughs in medicine and technology, you will need to find up-to-date sources. If, however, you are researching a historical topic, older resources may still be useful.
4) Coverage:
You will also want to examine the content of the source and how it fits your information needs. Identify its relevancy to your topic and whether or not it addresses the subject matter.