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 them in a tank. Difficult but definitely not impossible!

Interesting information, the original can be found here: Can you tell the sex of your Goldfish?

The way I sex my goldfish is by looking at their anal opening. My fish are around 3 inches including fins. My females have what looks like an extra piece of skin sticking out from her anal opening, where-as the males do not.

__________________
Never under estimate the power of an empty fish tank

You don't mention the male breeding stars at all....WHY?

__________________
J-Pond
http://j-pond.myminicity.com/

please click on the egg/baby to help it grow:

I think my large green guy is a male. as far i know, i have only owned one female goldfish and that was my lemonhead who passed away

I figured it out when my tank was covered in eggs!