Question

How do yo tell a fish is a boy or girl?

Answer 1

When male goldfish are ready to breed they get breeding stars on their gills/cheeks and on their front fins. They look a bit like white pimples. My male goldfish "Big boy" has breeding stars at the moment, if you like i could post a picture. But you can just google search images of male goldfish with breeding stars or whatever they are called. Female goldfish during breeding season or when they are going to breed will seem round, because their ovaries would be full of eggs. 

Answer 2

Look at your goldfish. Boys will tend to have a longer, thinner body than girls.
Examine the fins on your goldfish. A boy's pectoral fins will be more prominent and angular than a girl's, with white bumps to the front of the fins.
Inspect the gills. Boys will have white pimples on the gill covering.

Study the shape of your goldfish's body. The female's abdomen will have a softer or possibly lopsided appearance. The male's abdomen will be harder and more symmetrical.

Check your goldfish's vent. This can be found on the underside of the fish, just in front of the anal fin. A female goldfish will have a vent that is more noticeable.