Conductivity can also be spoofed. You must do a cohort of tests starting with weight and dimensions. Can do specific gravity as well.Modern counterfeits get past XRF. The US anti counterfeit taskforce recommends PM tools that test conductivity. Of course that test only tests the intrinsic metal and doesn't mean your bar or coin wasn't made by Chinese pocketing the numismatic value e.g. gold half sovereigns and silver Morgans.
Xray won't detect a well plated coin but will detect if metal proportions are just a little off e.g. British versus Arabian gold sovereign.