Doing this (especially for baby goldfish) is a really tough thing to do. You can usually tell the sex of a goldfish when its about 1 year old.
The female has a larger butt.This is the best way to tell a female from a same sized male.
The female goldfish has a fanned fin and the male has a fin that sticks out.
Males always chase females around the aquarium most of the time. So the chaser is nearly always male, and the chase is female.
When boy goldfish want to mate, they get white spots on their tail. If your goldfish has a spotty tail, it’s definitely a male. If it doesn’t, it’s a girl or a boy who’s just not ready yet.
The male will develop white bumps on their gills and the females stomach area will enlarge as she fills with eggs. Also, the female will have a slightly larger and protruding vent (anal opening). And they will also look lopsided.