There are topics about this already.
But yes, the differences between the goldfish genders can be noted by looking at the shape of the vent, males have an oval shape, while females have it rounder and slightly convex.
Another way to see whether they're males/females is to look at the gill covers. If they're completely smooth it's a female, while males develop little white dots on their gill covers and on the first ray on their pectoral fins.

You shouldn't even try looking at fish younger than a year, so leave the baby orandas alone. Because it's first of all, very hard to notice on them, and second of all, you need experience to see it in the first place.