... How can we determine the adequacy of the existing feeder conduits as EGC's?
Spool out a wire above ground from one end to the other, terminate it to the enclosure at one end, then measure ohms from enclosure to wire at the other end? Subtract the known resistance of the wire you spooled out, compare the resulting ohms to that of a would-be copper EGC. If they're equal or less, you're okay.
You need half decent estimates of the length of conduit and spooled out wire, but if the conduit is still good it should exceed the requirement by a significant factor so precision is not essential. Underestimate the distances to be conservative.
Maybe someone will suggest an instrument that does it by itself, but that would work. I suppose if you could temporarily take the lines out of service you can use one of the existing conductors instead of spooling one out.
I suppose a simpler way to start would be to measure ohms from grounded conductor to enclosure at the panelboard. It won't necessarily give you enough accuracy if the RMC is borderline, but if you get an open line to start off then you know you have a problem.