when/if one lays eggs it is female...the other if it fert the eggs its a male...thats the only sure way except to cut them open
i used to think it had something to do with the overbite or lack of as the male would have the larger mouth to hold the fry/eggs...after some time looking at many many aros it seems to not be provable...more or less like an overbite/underbite in a person