If I could put the glass into the liquid it would be fairly easy. But, isn't the glass stuck to the top of a flatbed scanner in this case? Makes it a bit hard to put it into a liquid without breaking the scanner.
So you get (scannerglass + fluid + gridsample), then measure the grid point distance (by taking as large a distance as you can across the grid) and compare to what you know it should be. The closer the grid measurements are to the actual dimensions the closer the index of refraction of the fluid will match the glass.