Besides the quick and dirty test I mentioned before, the Haynes manual for
my daughter's Honda has a slick test using a manifold vacuum guage. Warm up
the engine and note the idle reading on the guage. Rev the engine to about
2000 rpm and let it stabilize, then watch the guage when you release the
throttle. If the guage drops quickly and returns to the idle reading within
2 seconds, the exhaust is clear. If the guage appears to hang for a moment
or very gradually returns to the idle reading it indicates blockage.
Mike