I have had alot of success with all my goldfish questions and problems at this site, lots of helpful people. There is a whole section on breeding and sexing in the forum. http://www.kokosgoldfish.com/ Here is the forum link.
http://www.kokosgoldfish.invisionzone.com/forum/index.php?act=idx

I have always had better luck seeing the breeding stars on the males. Alot easier than the whole booty check :) Hope this helps, good luck.

if in spawning mode the male will have white ick like patches on the gills. Females are more round but in egg shaped goldfish all of them are... well round.

I hope you know how big goldfish can get?

The one in back is usually the boy.

best fishes
David
www.goldfishgarage.com