Find one with the morphological characteristics you like. Then see if the dragon is active, has good tail fat reserves, check the vent for liquid stool that has dried on the animal, has bright eyes, no swelling or mouth deformation. Watch for tremors or shaking. If possible ask the dealer to toss in a cricket. If it eats that's good, but if it just ate it may not be hungry so its not a perfect indication of health. If there are any sick animals in the enclosure with the one you like, I wouldn't buy it just to be a bit safe.
Thats all I could come up with now. I may remember others later.
(Edited) I usually edit all my posts. Darn dyslexia.
But look around the mouth and nose. If there is any crusting I wouldn't buy it.