quaith writes "It's not the way they dress, but the appearance of their face. A study published in PLoS One by Nicholas O. Rule and Nalini Ambady of Tufts University used closely cropped greyscale photos of people's faces, standardized for size. Undergrads were asked to categorize each person as either a Democrat or Republican. In the first study, students were able to differentiate Republican from Democrat senate candidates. In the second, students were able to differentiate the political affiliation of other college students. Accuracy in both studies was about 60% — not perfect, but way better than chance."