ok, so how do you know when your catalytic converter is going bad in your car. there are a number of ways you can tell. you just have to know the signs. here i am going to give a few examples of the most common problems you can detect.

1. you hear a rattle underneath your car: when you are driving and you hear a rattle under your car, you know your catalytic converter is broken.. why? because you see, inside the catalytic converter, is a material shaped like a honeycomb, when exhaust passes through, it creates a chemical reaction. i dont know all the science behind it, but i know it cleans carbon to water. when this honeycomb shaped material breaks, you have pieces inside the catalytic converter and thats why you hear the rattle.

2. blockage: if for some reason there is material buildup the honeycomb shape material inside the catalytic converter, the exhaust air will not be able to pass through and therefore, your exhaust will be restricted.
as you can see from this picture. this is how it should be. notice there are no cracks or blockage on mine: this is how it should be.
good catalytic converter

anyways, hope that helps. if you were told of another things to look for, please let me know so i can learn from you.