I cannot tell. From what I have read, it is tough to tell them apart and many people don't feel confident about it until the fish have reached sexual maturity and are showing different behaviors. The whole time I had mine, I never knew what they were. Though, somebody with more goby experience than me might be able to give you a better idea. The females will get a plumper appearance. When the males are ready to mate, their black bands will fade some, but that won't happen unless the fish are in the right mood/conditions to want to mate.