You'll need a way to observe o2 sensor voltages, be it with a scanner at the OBDII port or a good logging meter at the ECM terminals. The front sensor, after the car is warm and engine racing at around 2500 - 3000 rpm, should cycle from .2v to .8v roughly every 8 - 10 seconds. The rear sensor should be a nearly flat line, with a cycling frequency no less than around
30 seconds or so. This is a normally operating system. If the rear sensor matches the front sensor waveform, you have a cat that is not doing its job. If the front sensor matches the rear sensor waveform, you have a sensor issue. But, if the latter is the case you would (at least in the case of most Toyotas) most likely have an additional trouble code indicating a range/performance problem with the front 02 sensor. Thats why a 420 code alone usually indicates a converter problem. Barring all this you can also perform an oxygen capture test to measure the residual unused oxygen coming from the converter, but this requires a gas analyzer. An infrared thermometer can also be used to check for uneven temperatures across the bottom of the converter, indicating a problem. Also, there is no rule indicating which component will fail first. There is no justification in replacing an oxygen sensor (or two) just because converters usually last longer. Any real technician with half decent equipment should be able to figure this out in less than ten minutes.