If u bonk it with a hammer, and it thuds, it is usually original. If it has a hollow bonk sound, it is usually empty or coming apart inside.
If u have a non contact infrared thermometer, read the exhaust pipe in front of it, in the middle of the converter, and after it. If it's stock, the converter should be hotter while running and cool slower (be hotter)than the pipe as it cools.
TIMINATOR