There is no clear science to distinguishing between valid and biased content. However using common sense and backing up your web information with paper sources will help.

Check the address of the site out.

Is the web site http://www.aol.com/~joann/cancer.html or http://www.cancer.org?

Chances are "Joann" isn't trying to mislead or cause harm, however nothing in her web address would indicate she is a subject expert on cancer either. It is wisest to backup information from miscellaneous and personal websites with reputable ones. Cancer.org is probably an association or organization site whose background can be researched through the public library.

Look for experts in listservs, associations, and university sites.