As a dragon approaches approximately a year and a half in age- lets say 4.5- 6" snout to vent or 16 - 18" in total length, the secondary sexual characteristics begin to develop.
The males head will begin to get larger and wider and even more triangular.
The crest on the neck will begin to get higher, and the spikes much longer.
The spikes on his back will get longer,
You'll notice that below the spikes there is a ridge of bone or cartilage under the skin that points upward, just as the spikes do. I don't know if there is a proper term for this characteristic but I call them vertebral spikes. On an adult male these bony protrusions under the skin, below the spikes are approximately 1/2 inch long (1.2 cm).
Females do not have this at all.
The spikes at the base of the tail will get substantially higher, almost like a secondary dorsal crest
The femoral pores on the underside of the back legs will grow much wider, become darker and start to fill with a waxy like substance.
The femoral pores will feel bumpy compared to a females which will continue to stay small, smooth and indistinct.
As the male dragon approaches adulthood you should notice that his body is also more triangular shaped ... upward triangular, and that the base of the tail is much more triangular than that of a female.
An adult females body is more rounded or pear shaped (especially in the abdominal area), and her tail should remain rounded in appearance.
If your dragon gets to be say, 5 inches snout to vent or longer, and it's total length is 18 or more inches and you haven't started to see some of these changes in appearance you most likely have a female dragon.
Hope this helps :)