To spot natural selection at work, Pritchard and Bruce Lahn, also a geneticist at the University of Chicago who has conducted independent research in the same area, first look for places along the human genome to identify sites that show changes in some people but not in others. Then they look at the genetic material surrounding the changed part.
If the surrounding area looks very different from one person to the next, the particular change probably occurred a long time ago, because the general area has had time to accumulate other changes in the DNA. If there are not many differences in the surrounding genetic sequence, that indicates the particular change is relatively new.
Then scientists figure out how widespread that particular change is in large populations. Changes that are both new and widespread reveal the hand of natural selection -- since advantageous genetic changes will quickly spread through the population.
Almost worth visiting for the graphic. Actually, it's kind of cheesy. But go look anyway.