well the males will get breeding spurs on their gill covers and the leading ray of the pectoral fin. The breeding spurs look like little spots of ich.
I sex fish by their anal vent. But this is easier to do on bigger fish. I have trouble on anything smaller than 3.5" because it's just too hard to tell.
Females have a bit of an outie for an anal vent, and males have an innie.
Jessica
PS. Here is a great description with pictures!
Sexing Goldfish