I've stood in a room and watched gold kilo bars being chopped with a guillotine - that particular dealer said they cut *every* bar they buy back over a certain size. They've found tungsten inside kilo bars before. Only needs to be one in a thousand to make a loss.
There's other detection methods like ultrasonic testing that work great on a smaller scale, but this was faster when you're handling the volume they were, and everything is sent for refining and fabrication into new bars. Kind of a shame to watch mint PAMP bars get the chop, but if you were spending $50-60k on a single item, you would like to know it was authentic.