You can't; elephantnoses advertise their sex by creating electric fields of certain frequencies. If you have an oscilloscope, males "sound" one way, and females another. Since elephantnoses are highly territorial under aquarium conditions, you should either keep one specimen or, in tanks 250 litres upwards, groups of six or more. In smaller groups the dominant one will "jam" the electric field of the weaker specimens, eventually causing so much stress the other fish usually die.