Well, first off, let me tell you, even judges have gotten it all wrong, so don't be too hard on yourself, especially at first. I can generally sex at 10 days, but most people have to wait until at least 2-3 weeks. Of course, the older they are the easier it is.
If you are checking around weaning time, 5-8 weeks (I wean at 8, but if the mom has a huge litter, or seems to need a break, I've gone earlier), you'll do pretty well. Hold the bun with it's hind feet facing you, by the scruff. Cradle the bun on your holding arm, and with the other hand, push the tail down with your thumb, and the top of the vent with your forefinger. As you stretch the vent open in this manner, you'll either see a slit that extends to the anus, or you'll see a rounder opening, and most times a penis will poke out. Occasionally, males of 8 weeks (but more likely around 14-16 weeks) will have testes developing, soft hairless sacs on either side of the vent.
Some breeds, you can just tell a buck from a doe by looking at their features, but vent sexing is the only way to know for certain.
HTH,
Kim
http://www.blessedfarm.com