With goldfish, it's all about who chases who at spawning time. (Although curiously it seems that some go both ways - they can can be the chaser and the chased. I havent figured that one out yet.)

Females get slight bulges in their abdominal regions when they are full of eggs. Male body builds seem very slightly elongated compared to females. They go around at spawning time nudging the female to dislodge the eggs. Then a while later you may see baby fish. Fishy love is a very fine thing indeed! I was told that if that happens, it's a sign that conditions in your pond are good.

Hi
With GF it's a tough call unless they are actually spawning lol With Swords /platies very easy. The anal fin is fan shaped in females pointed on males Does require a view from the side to see this feature ,but from the top males are much slimmer.

The swordtails have been breeding like crazy. Yesterday I noticed that some of the larger ones were very pale, more orange than red. And it hit me: they're baby goldfish!

I guess I don't have to know which is male and which female, so long as THEY know.

You'll have to let the Goldfish take care of themselves. The Swordtails are easy. Even without the swords, their bottom fin (Gonopodium) is always pointed and sort of tucked up until they are ready for the right time. You'll see them moving that "fin" around quite a bit. Hah. Have fun... enjoy your fish.

Oh by the way, I'm now living in Michigan. My Home building in Hawaiian Paradise Park has been interrupted by aging parents.

A Hui Hou.

Male goldfish of breeding age often have "breeding stars" on their gill covers and/or the first rib of their pectoral (front) fins. These look like grains of white sand. They are calcium deposits.

If you tip the fish over and look at the vent, females will have an "outie" and males an "innie." Once you compare a male to a female, that's the best way to tell (short of observing spawning).

Here is a link that might be useful: Pictures here.