If you are talking about a "commercial" nitrous tank, look at the valve. It should be a CGA 328 with external threads and the pocket where the nipple goes will be around 3/8" (10mm). If it doesn't have a label saying medical grade and a lot number sticker, there is a chance it might be ":car" nitrous that is denatured with sulfur and definitely not for human consumption.
I don't know much about the ~500g bottles. Never saw one.
Internal threads generally mean flammable gasses and a larger pocket for the nipple will be oxygen, helium, argon or nitrogen. CO2 has a flat surface with a small hole in the middle.
There should still be a label on the neck of the bottle saying what it is.