At the base of the tail, just above the vent, there is typically two separated bulges. If you see two clearly separate bulges, as in the upper image, you have a male.

At the base of the tail, just above the vent, there is typically one half moon shaped bulge or no bulge. If you see one bulge,  as in the upper image, you have a female.

When bearded dragons are young, they very hard to sex (there are the exceptions), but develop clearer signs of gender as they get older. You should be able to tell by the time they are three to four months old.

The best way to hold your bearded dragon when attempting to sex them, is to place your dragon in one hand, facing away from you, place your thumb over the back to hold them in place. With your other hand lift the tail up to approximately 90 degrees and gently twist the tail slightly to one side or the other.  You don't want to hurt your bearded dragon.