It is difficult to articulate to students just how to decide if a source on the Internet can be trusted. Credibility operates on a continuum and depends on context. If I am having students write a biography on Martin Luther King, Jr., it definitely matters whether or not they rely on personal web pages or blogs with a slant vs. more well-known references. What if a student is reading a thinly or thickly veiled racist remark about Dr. King and doesn't realize it? How can I give students the guidance they need to distinguish writing that has an agenda from more credible sources?
I recently came across this excellent list of TIPS to help deal with this challenge.