How to Sex a Goldfish

Sexing a goldfish is an important part of its ownership. Determining the correct sex of your goldfish can help you name your goldfish suitably or plan breeding activities. Tell the sex of the goldfish early in its life so that you may properly care for it.

Instructions

Step
1
Wait until your goldfish are ready to breed. Telling a goldfish's sex is easier when a goldfish is mature and interested in spawning. 
Step
2
Look at the goldfish's overall body type. Males tend to be thinner and longer than female goldfish. View your goldfish from above its aquarium. Males have more symmetric bodies than females. 
Step
3
Note the goldfish's abdomen. If the abdomen appears swollen, asymmetric and soft, the goldfish is probably female. A hard abdomen indicates a male goldfish. 
Step
4
Check the goldfish's gills and pectoral fins. If hard, white pimples develop in these areas, the goldfish is likely male. Goldfish belonging to species with short fins can also have their fins studied to determine sex. Male short-finned goldfish have relatively longer, sharper and stiffer pectoral fins and leading rays than their female counterparts. 
Step
5
Inspect the goldfish's vent. A female goldfish typically has a vent that is more prominent than a male goldfish's; their anal fin's leading ray is also thicker than a male's.                              

Source: http://www.ehow.com/how_2146225_sex-goldfish.html