Found this post by Pamnock:
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For very young rabbits, the distance between the genitals and therectum is the easiest way to tell. Does connect very closely,where bucks have a littlemore of a space.
For the older kits, scissor the tail between your first and secondfingers and gently press in front of the genitalia to exposeit. Does will have a slanted "taco" appearance, where babybucks will have a tiny protruding round"donut".
Since it is very easy to bruise and damage the genitals, do take care in attempting to sex them at a young age.
Occasionally kits are mis-sexed even by the experts, so there's no wayto be 100% certain, even with older kits. Bucks with splitpenis (hypospadias) often appear to be does even up to 4 months of age.(The reason it is helpful to also observe the spacing between thegenitals and rectum).
Pam