Crawl under the car and look... or put it on a lift. Stock mid pipe is blatantly obvious, it'll always have turned brown from surface crud and has 4 cats. Aftermarket catback should be pretty obvious and from the sound of it you at least have a catback on there.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.