How do know when a catalytic converter needs to be replaced?
Can we assume you failed your smog test?
If so, please provide the results
Best to just paste a picture
Generally they are replaced when they rattle, get clogged or rust away.
Your car doesn’t have an extra oxygen sensor to read the cat’s effectiveness. Your smog test station does, as @db4690 posts, so if it has failed smog, that’s a pretty big clue.