I suspect my rear catalytic converter might be bad/clogged, how can I test since it does not have O2 sensors that will read efficiency like the ones in front?

I'm experiencing low MPG and no CEL light!

 

You can measure temperature of the converter with an infrared thermometer. Temperature on the outgoing side of the cat converter should be much higher (30-100F) than on the side where the exhaust comes into it. Another way to check if you have a clogged converter is to loosen the bolts between the Y-pipe and main rear converter to create a small gap in the exhaust. If the engine runs happier like that - your converter is clogged.