It is extremely difficult even for trained rabbit owners/handlers to determine the gender of a baby rabbit. Around three months of age you should be able to figure out the gender by following this process:
If you push down (with a slow, gentle motion) around your rabbit's genitals a boy's penis will look like a human pinkie finger with a circular dot in the middle, while the girl's vagina will look come up like a slanted triangle, almost like lipstick, and it will have a vertical slit in it. Also, you sometimes can find big lumps next to the boy's genitals and sometimes you can find nipples on the girl's tummy. However if these features cannot be found, you cannot conclude the gender one way or the other, only the presence of them can determine anything.