You cannot tell a bearded dragon's sex by size, waving arms or how often he displays a black beard. These characteristics are displayed by both sexes and both can grow to be the same size.
At birth and until they reach the age of approximately 3 months all bearded dragons look like a female due to the hemipenes that are too small to be properly exposed. However, a professional eye might be able to see the hemipenes at a younger age.
You can successfully sex your bearded dragon by looking at the area above the vent. Gently lift the tail at a 90 degree angle, just above the vent will either be 2 bulges or 1. If you see two clearly separate bulges (the hemipenes) then you have a male. If you see only one, you most probably have a female.
Here are photos of a full grown male and female:
Both sexes have femoral pores on the inside of the back legs. When they are adults the male's femoral pores are much larger than the female's.
Male Bearded Dragon Femoral Pores:
Notice how much larger the male's femoral pores are than the females.
Female Bearded Dragon Femoral Pores:
Notice that they are very small and not very obvious.
Photos supplied by Ash: Bearded dragons featured are Sloan & Kwayze. Thank you very much for your contribution.