It is possible to sex them but takes alot of time staring at the fish in the store. The males have a rounder jaw as percieved from the side- from the bottom of their mouth to the bottom of the chin, they also tend to have a longer caudal fin(the second from the head). The third deciding factor is to watch behavior in the tank at the LFS the males tend to stake out a territory and chase other fish from it(more than the females). At my LFS it always seem the females way outnumber the males. The final test is when you bring them home as the previous post mentions but then it may be hard to return the fish to the LFS. The EPARC.com(Eastern Pa aaquarium
reef club) site has a nice article with pictures showing the difference.