Thunderbird....you need to stop answering fish questions pal until you learn a bit more. You are dealing with live animals here and incorrect advice could kill them !!!
Goldfish are fairly difficult to sex most of the time. However, during the breeding season (March to July), females become noticably plumper especially when viewed from above as they "fill up" with eggs.
Also, the MALES (not females) will develop tiny white spots over their head and gills (don't confuse it with whitespot though) called tubercles. This is a kind of spawning rash and is totally harmless.
Spawning is possible, but unlikely in small aquariums due to the amount of space needed. However, spawning is very common in garden ponds.
A ripe female is chased into the shallows of the pond or where there is dense vegetation. She will be persued by 5 or 6 males who are relentless in their quest to fertilize the eggs.
Spawning can last several days and can occur several times during spring and early summer. Many of the eggs and fry (which hatch after 5-10 days depending on water temp) are eaten but if there is enough cover and hiding places, some might survive.
Hope this helps a bit