catalytic or heat shield?

[apb]

I have a rattle under my car, can't find an obvious/loose heatshield. I understand the cat can rattle - I have 79920 miles (so emission warranty is about to go). How do I check the cat (hit it with a wood block and listen for rattle)?

Hate to have a bad cat and barely miss the warranty.
Dealer cannot get me in before Monday, and I will be past 80kmi by then . . .