machine that basically x-rays the token or tokens. It reds not completly through but into a few layers of the metal. I x-ray both sides when I test tokens. The results can vary in different areas of exact composition if made of different metals. Like a copper nickle will read slight differences here and there. On this token it reads like the picture below. The rim will read 24kt and then the different metal under the gold plating.

This is why in my book when I mark as made of .999 or sterling I know it for a fact.

Hope this helps.