You cannot reliably tell a goldfish’s sex just by looking at it.
Why It Is Difficult
External appearance: Goldfish do not show obvious external differences between males and females like some other species.
Internal anatomy: The only reliable method is a veterinary examination to check internal organs.
Possible Clues (Not Definitive)
Males: Tend to be smaller and more slender; may develop breeding tubercles (small white bumps) on gills and fins during breeding season.
Females: Tend to have rounder bellies, especially during breeding season.
These are only general tendencies and are not always accurate. If you need to know the sex for breeding purposes, consult a veterinarian.