I'd say, that the only way to make any difference between a male and a female humanoid, is to look at the creature's physical appearance. If it is small and hasn't large muscles, then it must a female.
For example, in the Avernum -series(* the Nephil with a silvery or white fur and blue eyes seems to be a female Nephil. That's what I think, anyway. Just compare her to the other Nephils, and you see that they're far more muscular. The same goes for Sliths. There might be a slight difference of tone in the voice, as well, just as we humans have. Moreover, there might be a tiny scent, though we humans cannot detect it.
*) not the first Avernum game, though. Nephils not allowed in the party.