If you can't tell whether you have a male or female, it's probably a
female. :) Look on the belly of the ferret, about halfway between the
tail and the bottom of the rib cage.  If you see what looks like an
"outie" belly button, it's a male -- and it's not a belly button.
Otherwise, look just in front of the anus for a second opening,
perhaps with a tiny flap of skin.  If you see that, it's a female.

To double-check, look at a once-used litter pan.  Ferrets usually
urinate and defecate in one "sitting," and because of the anatomy
described above, males leave puddles a few inches in front of their
piles, females right on top.