Here are some ways to indirectly tell:
* Breeding season: During breeding season (usually spring), males develop bumps on their gills called breeding tubercles. Females also tend to become rounder and fuller due to their developing eggs.
* Behavior: Male goldfish will often chase females during breeding season and try to nudge them towards a spawning site.
* Internal examination: A veterinarian can perform a genital examination to determine the sex of a goldfish, but this is invasive and should only be done by a qualified professional.
It's important to note that even these methods aren't foolproof. Some goldfish may exhibit traits that don't match their biological sex, and some might be too young or old to display breeding behaviors.
If you're curious about the sex of your goldfish, you can observe their behavior during breeding season or consult a veterinarian. But ultimately, there's no way to be 100% sure without a professional examination.