As with most aquatic pets such as goldfish, turtles, and others,  you will not be able  to tell their gender until they are bigger and older. With goldfish, it is easier if they are at least 3 inches long. If you look at the tail, female goldfish will look like a bigger V than the male.  Also  for male goldfish, the area around the anal opening to the base of the tail fins will be convex and significantly curve up from point A. In female goldfish , the area will not cave as much. 

It easier to gender the goldfish if they are mature and in season. During spawning time, male goldfish  will have little white specks on their gill covers, not to be confused with ick, while female goldfish will not have these dots, but may look a little chubby around the middle because of the eggs in her body.

Males also have midline ridges on their undersides beginning from the back of their pelvic fins and ending at their vent opening, a feature which can be smaller or absent in female goldfish.
Males tend to have  longer and more pointed pectorals as well as stiff fin ray, while the females have more rounded pectorals and shorter, finer front fin rays.

During spawning time the males will be chasing the females so that's the more obvious way to gender the goldfish.