I've diagnosed this condition many times with a vehicle-specific scan tool, and I don't think it can be done without this or an oscilloscope. O2 sensors work in the range of less than 1 volt (.2v to .7v) In particular, the rear O2 sensor should stay at a fairly constant .5v if the cat is working right. Wild swings to the extremes indicate the cat isn't working.
The voltage fluctuates rapidly and is so small that a normal DVOM won't work well (it will average the readings). I suppose a analog meter with a small enough scale may work, but you'd have to pierce the insulation, as all of the wires have to stay plugged in for reference voltage purposes.
It's also possible that it's an intermittent problem, in which case historical data and real-time data while driving the car becomes important.
A gas analyzer would tell you about the efficiency of the cat, but that's even more of a hassle than a scan tool..
Edit: If you have two rear O2 sensors, and they have the same connector and wire lengths, you can swap them. Then reset the engine codes (disconnect battery) and drive it until the light comes back on. See if the error code went to a different bank. If so, it is unlikely that you have two faulty sensors, and the problem is the cat or something else up the line from the sensor.
You can also check the physical condition of the sensor and make sure there isn't corrosion/water/oil in the connector.