All the pages I read on green spotted puffers say that there is not really a way to tell. Many of the ways to tell are relative. Some say the females will be more dull than the males in color. That's not to say a male couldn't have dull color. Are you trying to breed your puffers? Breeding is difficult in captivity, but if you are interested I can look for all the best information for you. Granted nothing is guaranteed.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."