Watch for the behavior of the fish. Males tend to be doing the chasing when they are in breeding condition.

As a general rule,  the male goldfish will show the tubercles ( males can get them at 3-4 months old but can very depending on the individual goldfish) .
Male fish will get the tubercles when they are in breeding condition, which you can sometime see on the leading edge of the pectoral fins also in addition to seeing them on the gill cover.The males if they are old enough should have little white bumps in a line on the first ray of the pectoral fins . Checking the tubercles on the pectoral fins is a better way to tell if a fish is a male or female , since some orandas and ranchus with hood development tend not get tubercles on the gill cover or very few.

white tubercles on pectoral fins
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check out the white dots on the pectoral fin of this male
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If the fish is still very young, you can check the front ray on the pectoral fin. If the ray is very thick it's most likely a male.


Another way to tell which gender they is to pick them up with your hand. The female will have a belly that  will feel soft and jelly like if she is in breeding condition  while the male will have a stiffer belly.

vent-
female- narrow protruding slightly out(more so when spawning)
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male - wide and normal or slightly caving inward (concave).
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Two boys! Let the breeding commence!

Thanks for the pics nabi. I found out Jose and Orbitz are officaily girls. And the rest are boys.

I dunno coudn't find any.