Something that worked well for me was to disconnect the snorkel from the
mass air flow sensor and plug it to keep air in the engine.  I ran a shop
vac pipe into the tail pipe and sealed it with duct tape.  The shop vac was
set up to blow, not vacuum.  With the shop vac powered up, I then sprayed
soapy water on the exhaust with a small spray bottle.  In a very short time
the leaks will foam up.  Bob's technique of placing a rag over the tailpipe
didn't help me find the small leaks (and there were a few) in my custom
fabricated exhaust.  This technique is also nice as the van isn't running,
therefore no heat.