I have an answer to your question. I've raised rabbits off and on for many years and when my son was young, he showed them in 4-H. It may take a little practice to be able to perform this "procedure," but you'll get it in time. It would be easier to show you than to explain it, but I'll give it a shot!
First, get a hold of your rabbit by the scruff (the loose skin over his shoulders, just behind his neck). This will help to "stabilize" him/her so he can't wiggle around as much. Sit down and lay him on his back on your lap, so he's not just hanging by his skin in your hand. Then, with your free hand, use two fingers (I use my fore-finger and middle finger - same hand of course) and VERY GENTLY put pressure on both sides, right next to his/her genitalia. Keep pushing down until either a purple-ish/pink-ish colored penis pops out, OR, until you can see what looks like kind of a tear-drop shaped slit. If you don't see the penis pop out right away and you're sure you're doing the procedure correctly, then stop pushing before you injure your rabbit, cuz you have a girl! If there's a penis, well, then, obviously, you have a boy!
Please let me know if you have any more questions. (:
peace to you...always,