To tell if a rabbit is a boy or a girl, look at the genitals and look for the testicles especially, which are located on the underside between the rear legs and the base of the tail. There are two holes located in that area. The closest hole to the tail is the anus.
To determine whether a rabbit is a boy or a girl:
Step 1: Flip the rabbit over to look at the underside
Carefully flip the rabbit over, and look at the underside. If the tail is in the way, brush it aside gently.
Step 2: Look for the testicles
Since both the male and female genitals can look rather similar, the easiest way to determine whether a rabbit is a boy or a girl is to look for the testicles. The testicles in boys will normally drop when they are anywhere from 9 to 20 weeks old. The testicles may be hidden by fur, so close examination is required.