Great question with no easy answer. As a compiler of websites I usually check out the site before I post it. On my Evaluating Websites page I have some good sites that will help guide you. For example the first site on the right states:
Is there an author? Is the page signed?
Is the author qualified? An expert?
Who is the sponsor?
Is the sponsor of the page reputable? How reputable?
Is there a link to information about the author or the sponsor?
If the page includes neither a signature nor indicates a sponsor, is there any other way to determine its origin?
I always play on a site for at least 5-10 minutes to test for grade level appropriateness. I also look at the ads on the page for inappropriate content and I click on all of the navigation links to see where they take me.
Lastly, I go to work and check to see if it is blocked by our filtering system :)
Jerry has a good point to check for an author and who is the sponsor (i.e. is it a commercial sponsor with an agenda?)
Keep in mind that there are other ways of researching reliable websites - this is just a quick snap shot solution. First box - Whois look-up plus your ending (com, org, net etc..) will take you to all the background registration on the particular url. Don't get overwhelmed by the amount of information but when you dig in a bit you'll certainly be able to decipher who developed it and much more. Once you have the hang of it, it will be easy to teach to both colleague and student. Good luck. AT