The only way I know is to listen for calling and find out which one is making the noise. I have had four unsexed blue auratus for about 5 years, and finally decided they are all females, even though they have laid infertile eggs. Auratus will sometimes lay eggs without a male present. Although I think I'm pretty good at guessing male from female in other species, going by body habitus, toe pads, etc. depending upon the species, I guessed all wrong with my auratus. I would have sworn that at least one was a male, because it was more slender, but apparently, it was just a female that was more shy and ate less. I think that if you were sold a 1:1 mated pair, the only way to tell them apart is to watch and listen very carefully for calling. Any reliable breeder selling you a mated pair will mark their separate shipping cups with the appropriate symbols. The attached photo is of some of my females doing a bit of [bleep]-slapping--perhaps waiting for a guy to show up to provide a purpose for it.

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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus