Best way is to see if they provide references.
Any college student can put anything they want on their .edu site, and .org companies might have their own agenda skewing facts, i.e. greenpeace.org. .gov sites are the most reliable.
Any teacher that doesn't accept .com sources should be punched hard in the face. nature.com, britannica.com, newscientist.com, etc. And if you need drug information, erowid.org is by far the best source for information.
"Don't judge a book by its cover." And if you have any doubt about the credibility of a sites content, find more sites that say the same thing.
Finding credible sites: Start with your well established sites, and go through their links. If you're forced into using a general search engine, Google will return the best results. If possible, search using technical terms. For example, searching for "heroin" will obviously yield alot of crap, but searching for diacetylmorphine instead will yield alot of credible sites.