It's difficult, and no particular physical aspects are anywhere near 100% accurate.
I have identified a few pairs (that actually went on to spawn) by noting that some (mature/adult) males' dorsal fins can be somewhat pointed upwards in a slight arc at the very back end, whereas in the females it tends to be quite rounded and even, with no upward pointy end.
Far from 100% accurate, but a good guesstimate might be 60% or so.