Hmm. It seems to me that a Feruchemist ought to have a general sense of how much of an attribute they have stored in a metal, and thus how much a particular size of metal ought to be able to hold. If you're sensing a reduced capacity, then that would suggest the piece is lacking in purity. Someone experienced would then be able to tell whether a particular piece was usable.
There must be at least some synergy between Feruchemical and Allomantic mixtures, at the very least, or else Compounding would be a lot trickier.