How do you sex rabbits?

Actually, we had this same problem. When we bought two young rabbits from the pet store, the shop owner told us they were both girls. We then went to two different veterinarians, who both also told us they were both girls, but added that it was very difficult to tell at a young age (nearly impossible...like fish I guess). So we didn't worry too much...and ended up with 18 baby rabbits a few months later.
My advice? If you're thinking about buying these rabbits, spay/neuter them as soon as possible, because by the time they're old enough, it may be too late!
Hope that helps!