The only reliable way is to look at the breeding tubes when the fish are in breeding condition; females have a short blunt tube, while males have a longer thinner one.
All the other methods (the shape of the intestines, the shape of the body between ventral and anal fins, the shape of the head) are all no more than possible indicators, unfortunately.
If you can get some clear side on views of them I could have a go at sexing them for you, it is very hard but males tend to have a very slightly different head shape and the mouth is more "beaky" its hard to explain.