The aureii of Severus Alexander are all very valuable, and leave great temptation for excellent fakes to be made.
At THIS level of value, ALL aureii should be checked for provenance and XRF testing, suspect or not, when they come up for sale.
XRF testing looks for trace elements, not gold, which is the main constituent. A considerable data base for trace elements was established at (I think) Oxford University on a large range of low condition ancient gold coins, using Neutron Activation Analysis. It is against this data base that trace elements are checked, using XRF.
It is normal for all valuable paintings to be authenticated, when they come up for sale.