Researchers can watch the brains of test subjects. When you think about people like you, they report, a certain part of your brain lights up; when you think about people different from you, a different part lights up. This kind of bias is completely unconscious, present in people who are absolutely positive they don't have it and who are committed to treating everyone fairly (and think they do). But it affects our actions, from hiring decisions to proscribing medications to book purchasing. The higher the degree of prejudice, the more it affects behavior.
To me, all this was great news. It give us the opportunity to get beyond the defensive arguments of whether or not bias is present (though some people will hold onto that forever), accept that it's in all of us (each of us with our own unique pattern), and get on with what to do about it. It's not a question of if you have bias, it's what yours looks like.
As Banaji said, you can discover from testing that "the data are coming out not in line with my conscious intention," realize that "you are shaping your mind and creating associations," and take responsibility for, literally, changing your mind.
At the Project Implicit website, you can "assess your conscious and unconscious preferences for over 90 different topics ranging from pets to political issues, ethnic groups to sports teams, and entertainers to styles of music," (joining the 12,000,000 people around the world who've offered themselves as subjects).