Sexing goldfish is very difficult before they reach maturity. Sometimes people can tell by looking at the shape of the vent. Females may have rounder convex vents while males have thinner concave vents. The pectoral fins of males may be rather thick and stiff (compared to those of female goldfish) and with a more pronounced outer ray. Some people also believe that male goldfish have longer pectoral fins that do female goldfish.
The following pictures will help you determine the sex of your mature goldfish. Thanks to Bludusty, Mikroll and EbayJo who contributed excellent pictures to this project.
Breeding Stars or Breeding Tubercles
Male goldfish will develop breeding stars on their gill covers (aka operculum) and along the first ray of their pectoral fins when they reach sexual maturity.
Below are pictures of a male calico shubunkin goldfish with breeding stars on his pectoral fin and operculum (pictures by Bludusty):
Male with breeding stars on pectoral fin and operculum:
This picture of Mikroll's red capped orandas shows the stars and thickening of the pectoral fin of the male (compared with the female). The female's body is also more rounded than the male's.
When goldfish are ready to breed you can observe changes to their vent area.
Below are pictures showing the vents of two mature male goldfish. The male vent is longer and is indented (concave). This is a lot easier to observe on a mature fish who is ready to spawn. (Pictures by Mikroll and Bludusty):
Below are pictures showing the vents of two mature female goldfish. The female vent is not as long but it is protruded (i.e. convex):
It is the female that really 'controls' the breeding. A female goldfish (not ready to breed yet) will send out pheromones (a type of steroid) which will encourage males to produce the milt. When the female is ready to breed she will release another type of pheromone that stimulates sexual behavior in the male. The male will react by checking out the females, nudging them in the vent area as you can see on this picture (Picture by Bludusty). Males may exhibit this behavior before the females are ready as well, but it will be fairly intense once the female is prepared to spawn. Once spawning starts the males will chase and bump the female until she releases her eggs.
The End Result
The female should release eggs that will look like these (Pictures by EbayQueen).
Goldfish in tanks will eat the eggs just like the black moor in the video, so if you want to raise fry you will either have to remove the eggs or the parents. Here is a picture of a 19 day old goldfish fry (picture by Mikroll).