Certain gestures and hand movements. The way they play with their hair. Shoulders, hands, and feet are also a good indicator. When you've lived here long enough, you learn to tell these subtle differences. However, even if you are certain 99.9% of the time, there's always that 0.01 margin for error, particularly when you are drunk, as your senses are heightened and far more alert, hence more skeptical (at least in my case). Thus, sometimes you might mistake a chick for a dude, and end up regretting it later