A basic way of testing for plugged cats is to measure the manifold vacuum at 800rpms and then raise the rpms to say 2000 the manifold vacuum should be greater at 2000. If it isn't the cat could be the problem.
Measuring the ackpressure at the O2 sensor hole is another way. Shouldn't have more than 2psi at normal loading.
Another way is to just leave the O2 sensor out and see if the reduction in pressure forward of the cat makes a difference in manifold vacuum.
Bosch Master, ASE Master, L1
33 years MB technician