Summarize the following:
Sexing your goldfish is probably the single most important task in breeding; obviously, if you put a bunch of males together because you can't tell them apart, you're not going to end up with any progeny. Here's what females tend to look like:  Look at the shape of their vent. The vent is a small opening between the anus and the anal fin where fish excrete egg or sperm, depending on the sex. Female vents are rounded and convex, like an outie. Feel the abdomen. The abdomen, between the pelvic and anal fins, is very soft and movable for females. Look at the pectoral fins. The pectoral fins of females are round and short. Overall, female goldfish tend to be a bit smaller and rounder than male goldfish, who tend to be longer and pointier. This, however, is a less reliable way to tell the two apart. Male goldfish tend to be a bit smaller than their female counterparts. They can also be distinguished by looking at the following traits:  The presence of small white stars, or tubercles. Tubercles are little bumps that appear on the fins, head, and even gills of males when they're ready to breed. A concave, or innie, vent. Males have a vent that slices inward instead of bulges outward. Feel the abdomen. The abdomens of males are much more rigid and hard than those of females. Look at the pectoral fins. The pectoral fins of males are pointier and longer compared to the short, round pectoral fins of the females. During breeding season, males start chasing around females, sluggishly at first but with increasing fervor. Introduce a known female into the tank and look for the reaction of the other fish: Males will show a lot of interest, whereas females will show none! Many breeders separate males and females at least a few weeks before breeding in order to create a greater desire to breed. Just like with humans, absence makes the heart grow fonder!

Summary:
Know what a female goldfish tends to look like. Know what a male goldfish tends to look like. Look at the behavior of goldfish to tell apart males and females. Consider isolating males and females for a couple weeks before breeding.