What exactly happens if a cat gets clogged? Also, is there any way to check with "eyes" (after taking cat out of exhaust if the cat got clogged?

Plus, is there possibility of 4000 miles old factory cat getting clogged?

 

the honey comb inside the cat can look plugged, but it its plugged solid the engine will not even start. If the cat is just starting to plug up you will notice poor performance, engine will have a hard time getting its RPMS up