Put the car on jack stands, start it up in Neutral (or Park), and without getting under the car, listen for any pops or ticks along the length of the car. Any leak would be at the connection point between downpipe/testpipe, testpipe/x-pipe, or x-pipe/tailpipe. You might also see soot marks. Turn off the engine and once cool, tighten all bolts and possibly replace the gaskets.