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).
XRF will go a millimetre or so into the sample AFAIK - it's mainly useful for finding alloy contamination, e.g. "this is 99.99% gold" vs "this is 73% gold and 27% other things" - you really want to use both techniques. Resistivity/conductivity is about average composition, XRF is about elemental purity.
Huh, impressive - I guess I'm out of date (and/or the people I listen to about such things have a working XRF already and don't want to replace a working unit).
(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).