Goldfish can only be reliably sexed when they are breeding. Males will develop breeding stars, raised white spots on the gill covers and will begin chasing females. The shape of the vent will also change. Female's vents will become convex and male's vents will become concave. A lot of breeders say that goldfish tend to spawn around the full moon. Sexing Gold fish Generally goldfish will spawn any time they are kept in mixed-sex groups and have good housing and food. A lot of goldfish keepers I know have had surprise spawns. I like to keep all one sex to prevent any of that.
Do you know what you are getting into with both keeping goldfish as well as breeding them? Goldfish grow fast and large. You're looking at some large tanks in order to successfully grow out a spawn without stunting. I also recommend that you join a more goldfish-centric forum as I believe there is only one member on this entire forum (BettaFish.com and TropicalFishKeeping.com) who has successfully spawned goldfish.