On the individual scale, something like a Sigma Metalytics resistivity analyser (passes an electric field through the sample, checks to see if resistivity, conductivity, weight or size all match up to what you'd expect from a homogeneous gold sample of that size or weight). On the commercial scale, an XRF spectrometer. Both methods are completely non-destructive, and highly accurate (the XRF more so).
(the two scales are due to price and bulk: the Sigma is about the size of a hardback book and a few hundred pounds, an XRF is the size of a large 3d-printer and many thousands).
(the two scales are due to price and bulk: the Sigma is about the size of a hardback book and a few hundred pounds, an XRF is the size of a large 3d-printer and many thousands).