With all goldfish there are several ways to try and identify their sex!
If your goldfish is a male it will develop breeding stars on his gill covers and along the first ray of his pectoral fins when he reaches sexual maturity, it will look something like this
They will be white dots so care should be taken not to confuse them with ich or vice-versa
Another way to tell is when goldfish are ready to breed you will see a difference in their vent. The males will be longer and indented like this
while the females is not as long and will protrude like this
Less reliable methods would be to (after they reach sexual maturity) look at your goldfish from above as generally speaking females will be slightly more round than males
Also a breeding behavior of males is that they chase the females and bump in to their vent area, but again this is not always a reliable method
Here it and article on sexing goldfish: http://thegab.org/Goldfish/sexing-goldfish.html