Typically, a blocked exhaust (e.g. partially clogged convertor) is identified by measuring the intake manifold vacuum; the service manual should explain where to connect a vacuum gauge and what sort of readings should be seen during the test.

When the convertor clogged on my old '82 Cutlass, a friendly Car-X dealer unbolted the convertor/pipe assembly (from exhaust manifold to muffler) and let me test-drive the car without the convertor in place, just to verify that it was in fact the convertor that had clogged, before I spent the money on a new one.