This is what I was told to look out for in the male....breadth of dorsal fin, squarer corners to the caudal fin, long ventrals (at least 1/2 the body length) and a longer, more tapered body shape & Females tend to have a shorter, chunkier body, shorter ventrals, a smaller and narrower dorsal, a more rounded tail shape, a fuller belly (sometimes with the eggs visible as a pale patch) and an 'egg spot'.
hope that helps