There are several ways to tell the sex of a LH.
(1) The hump (kok) - USUALLY male fish have bigger humps on their heads.
(2) In the earlier breeds of FH, about 70% of fishes with black spots on the dorsal fin are females. However, with increasing cross-breeding, this method is not that accurate anymore. Also, do note that the double flowers of some LHs sometimes extend to the dorsal fin, again making this method unreliable.
(3) In a batch of fries, the ones that grow the biggest and fastest are usually males.
(4) The most reliable method is to look at the genetalia of the LH - the tube protruding from the underside of the fish (no that's not their ###### hole! It's their sex organ!

) Males have more narrow tubes with a V-shape opening. Females have thicker tubes with U-shape openings. It is only possible to see this when the fish is at least 2 inches. For smaller fishes, you may have to catch the fish, bring it out of the water, and examine it with a magnifying glass!
(5) The last method of telling the sex is to wait and see if the fish lays eggs!

When the female fish is about to lay eggs, its body color will darken considerably, it will lose its appetite for a few days, and its tube will be engorged and protrude out of the body prominently. If I dont remember wrongly, ZZ females start laying eggs as young as 5 months old, while JH varieties only lay eggs when they are over a year old.
Hope that helps...