Be wary of sites like wikis whose content may change rapidly and dramatically.
If you are looking at a website for which there is a print equivalent check to see if the entire work is online. If it is a portion of the work make sure that quotes have not been taken out of context or information has not been misrepresented.
You want to ensure your sources are at the appropriate level for your research, and distinguish between facts and opinions. What is the difference between fact and opinion?
Facts are usually verifiable. Opinions may be based on factual information, but evolve from the interpretation of facts. Most scholarly work will contain both; for example, scientists develop interpretations of data from several points of view successively in their writing. Each point of view expresses the implications of a different assumption. Think of these writings as the interpretations themselves (i.e., a record of the process of interpreting). That record of process is extremely valuable to you when you find and recognize it because it gives you models for your own thoughts (either to emulate or avoid).