To sex young rabbits, you have to get really personal with them! It may be easier if you have someone to restrain the rabbit, though with practice, you can learn to hold 'em and look at the same time.
Turn the rabbit so that the feet are pointing up. At the very back end (of course) you have the tail. Immediately in front of the tail is the anus. In front of that is a sort of fuzz-covered bump. Gently pull the tail back, toward the rabbit's spine. Press on the front of the fuzzy bump, at the point furthest from the tail. This exposes the rabbit's genitals. If what pops out is completely round, and sticks out the same amount all the way around, that's a boy. The female's vulva is more v-shaped, and stands up higher in the front. Also, there is a bit more distance (we're talking millimeters here!) between the anus and the penis on the male, and the corresponding parts on the female. It will (usually) be a few weeks more before the males' testicles descend (one on either side), after that, it shouldn't be hard to tell at all.
Rabbit breeders joke about rabbits changing sexes. They don't, of course, but we have all been embarrassed at least once by selling or showing a rabbit that we thought was one gender, only to have someone else figure out that it was the other.
What breed are they? If they're a dwarf breed at six weeks, I'd say don't bother, I can never see it, just too darned small! Its much easier to check when they are older, just be sure you recheck them before you sell or show them. Nothing like getting DQ'd at a show for incorrect sex!
I had a "doe" here that won two legs as a Jr, and the next time I showed her, as a 6/8, she got Dq'd for having "boy parts" I was stunned! Never thought to recheck her!