Probably fluorescence. There are amine- or sulphhydryl-reactive probes
available, which form fluorescent compounds with proteins. See for
example the Molecular Probes homepage [Only registered users see links. ] (no affiliation).
You'd have to thinly spray the steel part with a solution of the reagent
in a suitable buffer, wait for the reaction and then illuminate with
shortwave light (UV or blue, depending on compound).
Of course it depends on what area those 55 pmol (several ug if you
assume the MW to be about 100 kDa) protein are spead out. A reasonably
tight spot should be easily detectable.
Of course you'd have to clean the parts afterwards before you can use
them, but if testing of samples suffices, that shouldn't be a problem.
Note however that parts should be cleaned protein-free before
sterilisation anyway, as heat or cross-linking agent usually used for
sterilisation will make cleaning more difficult. So if protein
contamination (rather than microorganisms) are your problem, you should
re-design your cleaning process.