ok, heres the thing about sexing goldfish. its hard... and heres why...

1. they need to be in season in order to sex them (meaning ready to mate)
2. to get them in season, they need to be in an outdoor set up.
3. you can only find out the sex if they are longer than 4 inches.

but the way that you tell is the males have a thickened pectoral fin on the edge closest to the head. they also have tubercules, which are little white dots on the gill covers and along the thickened edge of the pectoral fin.

the females lack this feature, they just look like any adolescent fish with no tubercules or thickened pectoral fin.

oh, and here's a pic
Male shubunkin