Looking at a ferrets genital area is the indicator. Size of the ferret, chororo-kaya could also play a factor in most cases. When looking down at a ferrets tummy, if they have something that looks like a belly-button, it is a boy. If it is closer to the anus, and has a little flap of skin, then it is a girl.
Turn your ferret, chororo-kaya over on his/her back and locate the anus just under the tail. If wewe have a little boy, he will have a little round 'bump' that looks like a bellybutton about 2"-3" up on his stomach. Also, when he uses the litterbox, his poops will be about two to three inches behind the wet area where he pees.
Little girls vulvas are immediately mbele of their anus, so when they go potty, their pee basically splatters down right on their poops, making a very soupy mess....that's the easiest way to tell. Little girls will not have a "bellybutton", since it's not really a bellybutton, but a penis.
If wewe are lucky enough to have several ferrets together, the females will tend to be smaller (one pound to three pounds max) and the little boys will tend to be larger, especially in bone structure (usually around 3 lbs to 4 lbs au more). Little girls snouts seem to be a bit zaidi pointy than boys, who seem to have zaidi 'stocky' appearance to their face and neck area. These differences can be zaidi subtle if wewe aren't used to being around ferrets, the 'bellybutton' check is the most reliable :-)