If you really want to evaluate your bias (known or unknown) you should
try contributing to www.wikipedia.org . They enforce a Neutral Point
of View (NPOV), and if you post something biased then other editors will call you on
it really quick. When there are that many people editing you get
revised to NPOV pretty quick usually. Occasionally you might find
something biased out in some obscure corner of the content, there are
over a million articles after all.
I have found that working on articles there really helps you to notice
bias in what you write, as well as in external sources. Of course
bias has always been an interesting topic for me anyway, so I guess I
am biased in my comments about bias.