The P0420 code is typically only for the sensor after the catalytic converter. It is not an "O2 sensor bad" code, it is a catalyst efficiency code which is normally the O2 sensor downstream from the catalytic converter, but it can also be caused by a bad catalytic converter, vacuum leaks, etc.
You can test them with an O-scope. The voltage range should be between .2-.7 volts and if the cat is in good condition, it should stay pretty close to .5 volts after the engine has warmed up fully. You can try to use a voltmeter, but O2 sensors can switch voltage very rapidly and most voltmeters can't read the changes that fast, so it will average them out.