How do u know if a goldfish is male or female?

Question

and if you have a male and femals, do they breed or is there something special you need to do?

Answer 1

Although is it impossible to sex Goldfish when they are young and not in breeding season, the male is usually smaller and more slender that the female. In the breeding season the male has white prickles, called breeding tubercles, on its gill covers and head. Seen from above the female will have a fatter appearance as she is carrying eggs.

Answer 2

The female Goldfish are slightly bigger than the males and look
even heavier in the abdomen when they are full of eggs.

Male Goldfish develop white spots on their gills and pectoral
called "breeding tubercles" during spawning time.

Males have midline ridges on their undersides beginning from
the back of their pelvic fins and ending at their vent opening.
This feature can be absent or smaller in females.

Males have firm abdomen while in females; the area between the
pelvic fins and the anal fins is more pliable.

Male Goldfish have longer and pointed pectorals as well as
stiff fin ray, while females have more rounded pectorals and
shorter, finer front fin rays.

Male goldfish have smaller and more oval anal openings and the
anal fins are not as thick as that of the female goldfish who has
a larger and rounder anal opening with a slight protrude. The
anal fin is also thicker.

The spawning time is the easiest time to distinguish the boys
from the girls. The female opening will look larger and more
swollen as the eggs begin to ripen. She would look big and
heavier. Since fish reproduce through external fertilization, you
can make out when a male is ready by the white breeding tubercles and you can actually push out the milt through his ventral opening by running a gentle finger along his sides.

There is one more obvious, but kind of time taking way to make
out which one of your fish is male and female. Find out who is
chasing who - the chaser will be the male and the chased and
tired out one will be the female! It's the natural tendency on
part of the male goldfish to chase and nudge at a female's rear
part.

Answer 3

When they are ready to breed, males get white tubercles on their gills.
Females have vents that stick out more and you might see the ovipositor.

There is no other way to tell with goldfish. Females are not bigger, males do not have thicker fins.