How do you test ID for a gas like argon for which there’s no compendial ID test and the manufacturer says this is controlled by their industry by different fittings?

It may be very appropriate for you to define that as a special material, for which you are not going
to do any incoming tests because you know the reliability of the supplier, and it’s not necessary.
For a resin, it may also be very appropriate for you to declare that a special material.