There are two ways a converter can fail:
1.) It can become clogged.
2.) It can become poisoned.
There really is no "inspection port" for someone to view, unless opened by torch. Some may say there usually is a noticeable drop in gas mileage associated with a clogged catalytic converter. A totally clogged converter may cause a lot of known issues with one being, the smell of rotten eggs (theory). I can't say I have smelled this before, but maybe I can smell ****! who knows? A consistently high idle, or erratic idle issue, no hard acceleration issue, and many others. (Symptoms) Physically, you can hear them fall apart from the inside (rattle), or hear them coming apart from the welds over time is a known issue.