The only thing I can think of is that there are certain 'matting' materials that are applied wet to the surface of steels. Once they dry they are removed and can then be examined microscopically for- microsturcture, creep, carbides, etc. Itis entirely non destructive.
I know there are some fellas in here that have preformed these tests and can probably help you out with more specifics.
The asumption is of course that surface indications are volumetric. I would think that it would work well for the determination of W carbides.