It is not possible to determine the sex of a goldfish just by looking at it. Goldfish are gonochoristic, meaning they have separate sexes, but there are no external visual differences between males and females.
Here are some ways to indirectly tell:
* Breeding season: During spring, males develop breeding tubercles (bumps) on their gills. Females become rounder and fuller due to developing eggs.
* Behavior: Males chase females and nudge them toward spawning sites.
* Internal examination: A veterinarian can perform a genital examination, but this is invasive and should only be done by a professional.
These methods are not foolproof. Some goldfish may not display typical traits. If you are curious, observe behavior during breeding season or consult a veterinarian.