I think its possible to tell the difference. You'll need a top-side view of the fish. I think females will be slightly 'fatter' as she's got all the eggs.
There is no definitive way to tell. There are several theories but none that produce very accurate results. If anyone could do this with any amouont of success they would certainly be in high demand by breeders. Some of the theories are based on head shape and other body parts but in fish these vary quite a lot unless the breeding line is very pure. HTH