You can nowadays look at a 3D-plot of internal density in an X-Ray computed tomography. On the chance that somebody mixed up an alloy that has the exact same density and X-Ray density as gold, you can try X-Ray absorption spectra which depend on nuclear resonances of different nuclei, so would conclusively prove presence and amount of non-Gold material.

And you can do activation analysis, where you activate the ingot with neutron radiation such that Gold isotopes form and decay. You then measure the decay gamma radiation spectrum and look for decay lines that are not gold but another material. This is non-destructive, but will make that ingot slightly radioactive for a while, but nothing that a few weeks of patience to wait for the decay can't fix.