Is my exhaust cloged.

I recently found a long running (5 years 60 k) exhaust lead at the manifold since the rear stud was missing on my exhaust manifold. What is the chance that I now have a partial clog in the catalytic converter or the exhaust that is causing worse gas mileage. I have fairly new O2 sensors they were replaced 20k or about 2 years ago. How could I test all of this. Also how do I test my O2 sensors to be sure they are good.