Funman is quite right.
If you're worried about the front O2 sensor you'll need to watch it on a live scan, is it naturally going lean and rich (not just in closed loop, but is the car really warmed up too?).
You can test the front O2 sensor by creating a vacuum leak and seeing if it goes lean and the car responds. To test that it can go rich you can snap the throttle or put propane in the intake.
If a catalytic converter is going bad, the front Catalyst may not be getting hot enough for example. When it does, it drives the rear O2 sensor rich because it stores up O2 to burn the fuel well in the 2nd rear part of the catalyst.
Exhaust leaks can easily fake a P0420 on the 8th generation, I'm not sure if the 7th generation was setup to be too sensitive too, so be sure to check for those.
For example while the car is cold quickly feel around the exhaust when you first start the engine...it does get hot fast! You'd be looking for a leak prior or at the catalytic converter.
Basically a P0420 should set when the rear O2 sensor is seeing a lot of switching from rich to lean, you know, back and forth, but on the 8th generation, even though the rear O2 sensor is reading pretty steady, which should indicate a good cat, just because it goes rich or lean, across .45 volts too much in a short period, it will throw the code.
I had an exhaust leak after a new cat (I'm a new welder) and even though the cat looked great, due to a bit of switching the code got thrown for example.
I'll try to post up a link to a long conversation I had about P0420 with another member who got a lot of detail out of me ....can't blame him really . . . he really wanted to do all he could before replacing that converter.
...and you really should, even if you determine fast that you need a new one, is it just old, or do you have another condition that's making your catalytic converter go bad? Poor spark? Imbalanced injectors? It's been months and I'm still trying to get my injectors nice and balanced, hard on cheap aftermarket injectors that I could afford.
Here's that promised link:
http://www.toyotanation.com/forum/131-8th-generation-1998-2002/858114-catalytic-converter-4.html