How can you detrmine a source's biases? Why is it important to consider a source's biases?

Question

If possible learn the background of a source. Books and articles often include a brief background of each author. URLs often include the organization that has produced a Web site.

Watch out for the propaganda techniques described in these sites.

http://www.sourcewatch.org/index.php?title=Propaganda_techniques

http://library.thinkquest.org/C0111500/proptech.htm

If you're researching a controversial topic, read several different points of view.

It's important to consider a source's bias in order to ascertain the facts so that you can make up your own mind.

I see Ms Sue gave some good sites for further research.
The simplest answer that came to my mind, based on experience/observation, is that when a source has an agenda or bias they tend to suppress-disparage-ignore-downplay-etc. the arguments of other sources (particularly ones they consider"enemies"). In general, a biased source only tells and is only concerned- with one side of a story or issue, the one that favors them or their agneda. Think of someone trying to sell you a used car; do you think they will tell you everything that is wrong with the car -if they think it will effect your decision to buy? A seller would thus be considered a biased source for the car's condition.
Determining a source's biases is a much more difficult question though -especially in politics. Typically, trying to "prove" a source is biased generates more 'heat' than 'light' in my experience.
Does this help?

thanks i understand