I got this info from http://petcaretips.net/sexing-goldfish.html 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, females' abdomen 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. Hope this helps!

Male goldfish usually have white freckles beside their top fin. I've bred Goldfish for about three years now, and the male usually chases the female and nips at it frequently. The males also is almost always the one with a thinnish line that looks like a scar halfway across the center-top of the gills. Female goldfish have larger fins on just about every fin on the bottom, and they will look swollen if they're expecting eggs.

Male goldfish will be the ones nudging at the female under her tail, females will be the ones running away.