Not sure if I'll get an answer, but I figured I'd try:

I know Corsa mufflers were known to fall apart internally. Before I consider spending money on quieting the exhaust, I want to be sure that mine is still sturdy (last year at the inspection station I heard metallic noise that sounded like it was coming from the muffler, but it could have been something in the shop: I haven't been able to replicate the sound).

What symptoms or things can I do to check if it's junk?

 

Corsa has a lifetime warranty.

 

I'm not the original owner

 

I had this happen on my original Corsa muffler, but the replacement has been flawless. There really weren't any early warning symptoms. One day, my old muffler let a baffle go, and there was a constant light metallic rattle. It was not intermittent for me, and it did not get worse after a few months, but it was annoying.