[flimsypremise]

Yes, there's actually a very good test for a properly inverted color negative. You need a negative of a greyscale step scale from light to dark. If the color channels are properly linear relative to one another in the scanned image, you should be able to white balance any one of the patches and have all of the others remain neutral: by which I mean each patch should have equal amounts of red blue and green. In practice the characteristic curves of film often mean the darkest and lightest patches are often slightly more green or red, but its very close.

So the process would be, using the RAW scan of the image (the orange mask intact):

1) Invert the image.

2) White balance on any patch.

3) Sample the color balance of every other patch. They should have equal amounts of all colors.