Well, IIRC the krogans & salarians are evolved reptilians & amphibians respectively, and the two sexes of these animals generally look alike (beyond occasional difference in size depending on species). As for turians, I don't see how males & females could be made different as their face is dominated by a metallic exoskeleton... beyond the exoskeleton having a different complexion depending on gender.
As for the other races that look like jellyfish & sloths (forgot the name) - hard to make big differences between the genders here, beyond giving them another colour and/or size.
Quarians & volus - can't comment as the ones found in the game wear an environmental suit, so who knows...