There are a few different ways documented of sexing Bearded Dragons, though only one that has proven to be most effective. The most effective way is to attempt to view the hemipenal bulges and the lack there of in females. Sexing hatchling Bearded Dragons can be tricky as they are so small and the hemipenes are harder to expose. With a Bearded Dragon that is 4 or more weeks old the process is much easier.
To safely and effectively expose the hemipenal bulges, place the specimen Bearded Dragon in the palm of your hand or on a sturdy surface where the body and legs of the Bearded Dragon will be supported and with the tail facing you and gently bend the tail straight up and look just above the ventral opening. In male Bearded Dragons you will be able to see the hemipenal bulges on both sides of the base of the tail, which will cause and indentation between the two hemipenes, where as in females the hemipenal bulges will not be present. Due to the lack of hemipenal bulges in female Bearded Dragons you will notice a single bulge centered just above the ventral opening.
Notice the two hemipenal bulges and the centered indentation in Males (left) and the lack of them Females displaying only the single centered bulge (right).
Another way to sex your Bearded Dragon, though really only works well on adult Bearded Dragons is checking for femoral pores along the insides of the rear legs. Both sexes have them but as adults the males femoral pores will be enlarged and a lot easier to see whereas in females they stay relatively small and harder to see.
Notice enlarged femoral pores on the insides of the Males rear legs (left) and the smaller less visible femoral pores in Females (right).