Mazda Repair: Code P0421 on 1998 Mazda Protege, catalytic convertor, o2 sensors

Question
I just bought a 1998 Mazda Protege 1.5 LX about two weeks ago. Since then I have changed the oil and air filter, changed the oil, checked the plugs and wires (no need of tune-up), and I'm in the process of changing the coolant (waiting because cap to overflow container is busted, going to junk yard to get a new one). My check engine light came on about 3 days after purchasing and after running engine diagnostic for the codes a P0421 came on. I know this either has something to do with the cat converter or the o2 sensors. How do I tell which one? I don't know if there are symptoms of each but here are some things I've noticed: the engine shakes while idling #after getting two new tires mechanic said their is a bent rim on passenger front side and that it would cause it to vibrate) BUT what about when the tires aren't moving?, another thing is I smell some sort of gas or something close to it occasionally. I haven't found a pattern as to when I smell it. Last thing, sometimes when I start it the car makes a screeching noise for about 30-45 seconds and then stops. It becomes louder when I accelerate. I have notice it only does this when it's kinda cold out.

Q: Is there anyway to tell whether to replace the O2 sensors or the Catalytic Convertor? I would hate to replace one and find out it's the other, or replace both and only need one.

Thanks for reading. I really appreciate, this is my first car and I don't really know much about them.

Answer
the cat is the easy one to test but this only tells you if it's cloged not if it's working right there is not true way to test the cat unless you have emissions equipment. by vacumm testing the engine that'll tell you if its cloged or not. to test the o2 sensor you'll need a wiring diagram and a reference chart and a good multi meter test resistance and compair to the chart if there not within spec you got a bad 02 sensor. sorry but i dont have that info off hand. as for the shacking try adjusting the idle if it can be. and the sqeaky belt try delt dressing or a bar of soap to see if that help if not replace the belt.