Just start the car and listen. Seriously though, the best way to find an exhaust leak it to get yourself a 2 foot section of hose. It can be lot's of different sizes, however 3/8 will work well. With the car running, hold one side to your ear and probe every connection on the system with the other end. Manifolds, collectors, clamps, etc., anwhere there is a gasket or connection. I garantee you will hear the difference if you have one.