Last night in Berlin's Olympic Stadium, an 18-year-old runner from a South African village received a gold medal for the 800-meter dash. But today, doctors are looking at test results to determine if Caster Semenya has too many male characteristics to compete as a woman. Some of Semenya's competitors say they can tell she's not really a woman just by looking. In most cases that's probably true, but medical experts say that for one percent of the population it's just not that easy. David Epstein is a staff writer for Sports Illustrated.