How do you tell if a goldfish is a girl or a boy?

Best time to tell is during their spring breeding season-
the females become fatter as they fill with eggs to spawn, and the males develop tiny white spots on their head and gill covers.
Other facts:
Females are slightly bigger , heavier in abdomen when full of eggs
Males abdomen are firmer, females more pliable
Males have midline ridges on their undersides
Males longer pointed pectorals, stiff fin ray
Females more rounded pectorals, shorter & finer front fin rays
Males- smaller and more oval anal opening
Female ? larger, rounder anal opening w/ slight protrude. anal fin is also thicker
Males chase and nudge at female?s rear part ? ok too much info here :-)

Sexing goldfish is very difficult before they reach maturity. Sometimes people can tell by looking at the shape of the vent. Females may have rounder convex vents while males have thinner concave vents. The pectoral fins of males may be rather thick and stiff (compared to those of female goldfish) and with a more pronounced outer ray. Some people also believe that male goldfish have longer pectoral fins that do female goldfish.

i have no idea i need help with the same question

you should probably wait until they mate males develop white spots on their tail fins and girls obviously lay the eggs. Also females are bigger than males seeming they have a bigger abdomen for eggs.

the female goldfish is rounded and the male`s have a narrow "ish" head.