Please note: Photos on this page may be disturbing.
First things first, make sure the rabbit is comfortable because an uncomfortable rabbit on its back will kick and scratch until it is put right-side up. To do this, place your index finger between the rabbit's ears and grasp the base of the head with your thumb on one side and your other three fingers on the other. With your other hand, either cradle the hind end.
Both males and females have genitals that "pop out," so they can be easily confused. The opening closest to the tail is the anus. The opening closest to the belly is the genitals. Gently place a finger to each side and lightly apply pressure down. Examine what pops up. Sometimes the male genital pops right out. Other times, the male genital does not appear, but you see a tube with a small opening at the top. This is a boy. Girls have a slit, that is an opening that starts at the high point and ends where the genitals meet the body, near the anus. Sometimes the area appears to have some dark red coloring, and other times it is pink. What sometimes confuses people is that "something"- the vulva, pops out and there can be some height. Don't let this confuse you: if there is a slit you have a girl.