Look at their dorsal fins. The first two spines are longer on the male. Males also show more color and grow larger. Females are stockier, plumper, and have blue spangles in that black blotch. Females also get a faint pink blush on their bellies at spawning time. Most of the rams that show up commercially are males.
http://aqualandpetsplus.com/Cichlid, Rams.htm
http://aqualandpetsplus.com/Cichlid, Rams.htm