I don't usually try until the bunnies are at least 3 weeks old. Until then, the "parts" are so small, it's really hard to tell what you are looking at. The males' testicles usually don't appear until at least 6 weeks (some a lot later), after that, it's hard to miss! We rabbit breeders joke about bunnies changing sexes; we have all been embarrassed by selling/showing at least one that we thought was one gender, only to have someone else realize it wasn't what we thought!