I was just helping Graham on another thread and thought that this might be a good tip for those who can't work out the sex of their fish.
Goldies under 12 months of age are close to impossible to sex, so this list wont help much in this case, but as fish mature, they are a lot easier to work out.
I'm not saying this is a definitive list and it wont work 100%, but in general these are good pointers to look out for when trying to determine the sex of your goldfish. The only 100% way to know is to see females laying eggs. (Males, well, they're more obvious with breeding tubercles and all.)
-thinner whispier first ray on pec fin
-thicker first ray on anal fin
-less pronounced ridge than runs between ventral fins and anus
-if full of eggs tends to swell more on one side of abdo (best viewed from above)
-no breeding tubercles.
-an 'outie' cloaca/anus, kinda roundish...
-In personality I have found to be more antisocial
-gets chased by males.
-thicker first ray on pec fin
-thinner first ray on anal fin
-more pronounced ridge b/n ventrals and anus
-if in season/older than about 3yrs will show white spots on operculum and roughish white spots on the first ray of pectoral fins (feels rough to touch)
-And and 'innie' cloaca/anus -looks more like a slit
-Tend to be more rambunctious, likes to school more, will chase females that are 'on heat'