Here are some links with additional information on thinking critically:

http://www.coun.uvic.ca/learn/crit.html

http://falcon.jmu.edu/~ramseyil/critical.htm

http://www2.sjsu.edu/depts/itl/graphics/main.html

These links deal specifically with thinking critically about material you find on the internet:

http://www.lib.msu.edu/link/critical.htm

http://library.wlu.ca/critical/

http://www.library.cornell.edu/olinuris/ref/research/webeval.html