How can you determine a source's biases?

You have to do some research. Sometimes it takes a while, as it is not obvious. Bias often takes the form of legality, in sanctions, sentences, and other official act. The root cause may be buried in deep personal events long past. Evidence is diverse: other actions, speeches, writings, hirings, firings, etc.

The two most obvious biases that are easy to research are political and religious. I have always told my debate students, always check to see "whose ox is in the ditch" before using that person as a source. Good way to think! Please repost if you need further help.

"Bias' means "leaning" or "slant." Read the piece carefully and see whether the information given "leans" one way or the other. If it does, then there is bias. BuruBlue's test is a good one, too. By discovering "whose ox is in the ditch," you can see the bias for or against rescuing it.