Its usually fairly easy to sex rabbits by looking at their genitalia.
Its best to have the bunny either in your arms on it back, or with its back feet resting on a surface and the forelegs held up, so that the genital area can be visualized easily without the bunny struggling.
You then need to very gently exert pressure on either side of the genital opening- with a male rabbit- you will see a definite circular shaped opening, and sometimes the penis will protrude, when they are 12 weeks old testicles should also be visible on either side of the genital opening.
With female rabbits, when you look at the area and gently exert pressure, an obvious slit will be apparent, rather than a circular opening, and no penis will extrude.
Sexing rabbits is an acquired art, and is easy with practice, if in doubt take them in to your local vets- and the vet techs will be happy to sex them for you.
here is a link to an article with pictures