Theres no way to tell when looking at a single fish.....but as CFC mentioned you'll definately know if they were side by side.
I had bought two as babies at the same size........after about a year my female was about an inch longer and about a half an inch wider from dorsal fin to pectoral fins and much thicker. My male whom I still have is more slender and looks more like a big baby bala than the female did.
Also not really revelant to the topic as you can't tell when they are babies, but my female was alot more skiddish than my male.