courtesy of Jeff Kubina
The only sure way to distinguish the gender of a freshwater angelfish is to observe their spawning equipment, which doesn’t get deployed until they’re ready to spawn. Before that, you can make some educated guesses based on physical features.
Although there are no hard rules, female angelfish are usually smaller than males. If you want a pair and plan to buy some juveniles, pick three larger fish and three smaller ones.
In several months when the angelfish are close to adult size, the females will become gravid. When they are swimming toward you, look between the ventral (bottom) fins. The females will usually have a larger tummy than the males.
The third way to make an educated guess is to draw an imaginary line vertically near the gill plate to the top of their head. Now look from that line to their upper lip. Females usually have a straight forehead, and males have a forehead that drops just above the eye then continues fairly straight to the upper lip. They call that kind of description anthropomorphism, but I thought it would be easier to understand than a bunch of Latin words I don’t know