Get the car on a lift and inspect it. If it's a header gasket leak that should be relative easy to determine, but if the leak is at either end of the mid-pipe you may find it easier to locate if you get the car up in the air with you under it.
If it's coming from a blown header gasket, you can take a piece of paper and hold it near the headers. If there's a leak it will blow back-forth rapidly with the exhaust notes.