Getting access to the exhaust manifold to check for leaks

I'm trying to chase down the cause of a P0420 code and make sure there isn't anything else causing it before I take the rather hefty financial hit of replacing the cat. Since it's the first Prius I've ever even been around when running I can't tell if its exhaust makes more noise than it should, but it has always seemed to me that it makes more noise up near the engine than it should, so I'm planning (well, 'fogging' is probably more accurate given my plan...) on smoking the intake and exhaust. The intake is fairly easy- but the exhaust, the exhaust manifold is rather buried. Do I just need to take the cowling off so I can look down at it and see if it's leaking anywhere? I'm assuming I won't be able to see it completely from just down below it...

Thanks!

 

264k, and actually I don't recall it being very rusty at all.