I jacked up the car and placed on jack stands. saftey first. Leaks in the system generally would show at the joints in the system. I would look for carbon buildup along the entire lenght of the system. The carbon would indicate a leak. Also check the pipe for rust and weak spots. Removing an upstream O2 sensor as noted above is a good way to check for excessive back pressure. Direct fit cats are available if that is your issue. 4 bolts and the job is done. You may want to have new bolts available in case you break them like I did. If you are not comfortable with checking your self have a quality exhuast shop check the system for you.
