Websites present a unique problem for evaluation in that anyone can create and post a website. If you want to use a website for information, and especially if you are going to cite a website in a paper, you must make sure that the website is a credible source.
Despite the somewhat silly and simplistic acronym, one good method for analyzing a website's credibility is the CRAAP test. It goes as follows:
(This list is paraphrased from the CRAAP test document produced by the Meriam Library at California State University. Click on the hyperlink for the full document)
There is no one exact way to determine whether a website is a credible source; you need to use your judgement. The CRAAP test gives you a list of useful questions to go through, and is one method for making the evaluation process easier.