On a breed of rabbit as small as a Lionhead, I don't even try until they are at least 3 weeks old (the parts are so small and my eyes aren't as sharp as they used to be). When I do try, I turn the bunny on its back in one hand, and gently pull its tail toward the spine. Just in front of the anus is a fuzzy bump. Push gently on the front of the bump (the part furthest from the tail). This will expose the genitals of the bunny. If what you see sticking out is round like a doughnut, with the hole in the middle like a dot, that's a boy. If it's more or less v-shaped, with a slit for an opening, that's a girl. As the bunnies get older, you may notice that there is a tiny bit more space between the male's "parts" and the anus than with a female's; also the female's vulva sticks up more at the front, tapering down to flush with the surrounding tissue at the back, while a buck's penis sticks up the same amount all the way around.
With really tiny bunnies, there's an informal way that I learned a long time ago. You've probably noticed that the little squirts are bad about peeing when you pick them up. If the pee dribbles down the tail, it's most likely a girl. If it shoots you in the eye, it's a boy!