There are several methods of determining the sex of your fish but it is still difficult for most people to be sure till the fish attain the breeding stage.
Male goldfish are easier to spot than the female during spawning. They develop tubercles or white pimples along the pectoral fins and gill operculum. The female become a little asymmetric, especially around their abdomen. They look swollen in this condition.
Post spawning and after a few spawning seasons, some males retain roughness on their pectoral rays. It is difficult to spot this roughness and people who feel confused should take solace in the fact that not many people can distinguish between male and female goldfish!
Following are some methods that can be used to recognize the sexes but even these are useless unless the fish are at least a year old, i.e., have attained sexual maturity.
1. Midline ridge: male goldfish have a ridge running through the back of pelvic fins to their vent opening on the underside. The ridge is either completely missing or smaller in females.
2. Firmness of abdomen: The area between pelvic and anal fins is pliable for females but very firm for males.
3. Shape of the vent: Though difficult to figure out, the female vent is rounder and convex, whereas the male vent is thinner and concave.
4. Shape of the pectoral fins: Male pectoral fins are pointed with stiffer leading ray and female pectorals are rounded, shorter with finer front fin ray.
5. Features: females are brighter colored and are more active. This could be a difficult way of pointing out a female but you could try!
6. Check out: One theory says that you can introduce a known female into the tank and watch for the reaction of fish. Males will check out the new fish but the females will show no interest!
The only way you can learn to spot male and female goldfish is by closely observing known males and females and noticing their various traits. A pet shop could be a nice place to learn this.
During spawning season it will be easier because the male will follow and nudge the female relentlessly sometimes even bruising and injuring the female. Finally, it’s only your experience that will give you any degree of confidence of judging the sexes by looking at the goldfish in a tank. Difficult but definitely not impossible!