There are a number of checklists that have been developed to help you evaluate websites. One such checklist, the CRAAP Test, was developed by Meriam Library (California State University, Chico) and delivers a comprehensive list of questions to guide you the evaluation process. Upon completion of the worksheet you will have a final score that could range from "highly questionable source" to an "excellent source for research."
The CRAAP worksheet is included as a pdf below.