It's hard to sex beardies at a young age, but you can try. I could tell at 6 weeks that mine was a girl. You have to be very careful when they are that young though, since they are so fragile. I only recommend doing this if you are experienced with handling these little guys.
Hold him/her in your palm with your hand flat, fingers facing out. With it's head facing outward in the same direction as your fingers, very gently lift its tail upward, only until you can see the vent and the base of the tail. At this age, you will most likely see nothing, but you may also see a pronounced lump at the base of the tail, right in the middle, or you will see two longer "sticks" going upwards along each side of the tail and nothing in the middle. The 2 sticks is a male, middle lump is a female.