Goldfish are easy to sex if you can look at them out of the pond, like in a clear plastic container, or even laying on their side in your hand. Female goldfish have a protrusion at the vent. Males are more convex. Males will also get tuberacules on their gill plates when they are in spawning condition. This looks like little white dots. They will also get them on the leading rays of their pectoral fins.
Double tailed varieties...fantails, orandas, moors, ryukins, etc., etc., have double anal fins....or should. Sometimes, they don't, but they should. Single tailed goldfish...shubunkins, comets, common goldfish have single anal fins. As do koi. By way, shubunkins are single tailed goldfish, but if it is a double tailed calico goldfish, it is a calico fantail or calico oranda, or whatever.