According to FishChannel.com, determining the gender of a goldfish can be difficult, as the fish are weakly dimorphic. However, depending on when the fish are checked, it can be possible to determine their gender based on physical differences.
FishChannel.com states that determining the gender of immature goldfish is especially difficult. Females are typically larger and rounder than their male counterparts, while male goldfish usually have a thicker pectoral fin. Perhaps the easiest way to identify the sex of a goldfish is to examine them at breeding time, which is generally in the middle of the spring. During this time, female goldfish have bloated bellies, and male goldfish have white bumps on their gill covers.