Dragons are somewhat difficult to sex, but there are ways. First off,
you can't tell from the size of the head or the size of the body. The
Bearded Dragon I saw with the largest of both was a proud mother, not
the male that those attributes supposedly signal. It is also hard to
tell before around 6 months of age. Not impossible, but hard. Here is
what you are looking for: a) Femoral pores. This is on the left
picture. While both sexes have these, the male has much larger ones.
The picture shows a male. b) bumps around the vent area. One bump is female, two bumps is a male.