When a female angelfish is ready to breed, she and all male tank mates will drop their papilla. The papilla is a tubular structure located between the anal fin and ventral fin. The papilla of a female will generally be smaller and more blunt tipped than the male papilla.
Another way to determine the sex of an angelfish is to compare the general body shape of the fish. Male angelfish have a more angular body compared to the rounded body of a female. Some males are unique to females as they will form a bump on their head after they are a year old, according to KoiAngelfish.com.
Another way to spot the difference in gender is to look at the angelfish's dorsal fins. Female dorsal fins are held slightly backward, in line with the forehead angle. A male dorsal fin will stand fully erect at an almost 90 degree angle to the head bump. Looking at an angelfish's ventrals is also a good way to determine the sex of the angelfish. Female angelfish will hold their ventral fins closer to the body, whereas the ventral fins on a male will be held erect and create a distinct angle to the body.