It is a misconception that female goldfish have white bellies. This is not true.
Belly color is not a reliable way to determine sex. Goldfish come in many colors, and belly color varies widely among individuals regardless of sex.
The most reliable indicator is breeding tubercles (small bumps) on the gills and fins, which are typically more prominent in males during breeding season.
If you need to know your goldfish's sex, consult a veterinarian or experienced breeder.