What's needed is a mining version of the SRV scanner that allows you to locate high ore density pockets in the ring (like POI's on planetary surfaces) once there fire off prospector drones and assess the content of the rock.
@ Almont I agree that you should be able to determine the mineable prospect of a system by it's description (detailed planetary information is available after all) assuming it's been detail scanned.
I would add to that a “prospector” SLF that would scan asteroids unintrusively and mark them for you. You could do this with the assistance of multicrew or with NPC crewman. Wouldn’t that be nice?