Hi,
This may be difficult with a good fantail,
If you look just behind the anal fin you will be able to discern a small protuberance - what is called the 'anal pout' on a female goldfish (a rudimentary ovipositor)
the male goldfish lacks this entirely
This is most easily seen on larger females and females who have already spawned. Once you have seen it once you will know what to look for
You may want to go to a LFS to do some research on the specimens in the tanks there