[header] How to tell if a goldfish is pregnant [title] Find out if your fish is female. [step] The best way to find out the sex of your goldfish is to ask when you buy the fish, or to consult a vet. That said, females goldfish typically have plump bodies. 
When viewed from above, you can usually notice fuller bellies on the females, and slender midsections of the males. Also, the female pectoral fins (located just behind the gills) are shorter and rounder than those on males.