Exhaust Part Number?
I think the easiest way to tell would be to look at the driver side midpipe. If it is completely round, it is the catback. If it has flat spots in it it's stock. I think it would also be 2 1/2" catback vs 2 1/4" stock, but the easiest thing would be to look for the flattened areas.
2012 Grabber Blue V6 Convertible
1995 Red Trans Am Convertible
1979 Gold Trans Am