get your car up in the air, when its cold, start it get under it and feel for air blowing out of the pipe or res or muffler, start at the front, it heats up fast.
if your car has the stock exhaust on it still you will need to get stuff re welded in or if you can weld it yourself, thats a plus.
if its all clamped in and had been replaced its easy to replace whats rotted.
i found it easier to just drop the whole cat back, remove it from the car to work on it, 2 bolts at the cat and the rest is just rubber hangers. the mufflers have 2 bolts to drop the bracket that holds the rubber hangers, i took them 2 bolts off, and the muffler dropped real easy. they are the only hangers that may give you a problem.
plan on picking up a new cat to res gasket and 2 new bolts for it, they will break or strip out. truth be told its better if they just break, they never come off. i had to cut one off with a grinder as it stripped out the heads of the nut and bolt real easy. the other one snapped.