I find it easier to polish one small spot, and do the depth test. For transparent materials, the apparent thickness compared to an equal thickness of air is thinner. A look through a very thick slice of glass tipped at a large angle shows this effect very clearly. Measuring the apparent depth requires at least a small spot on one face be transparent. The actual measurement is very easy with a microscope, either a cobbled together fixed-focus one, or something fancier.
Micrometers are best, calipers a good second choice.
Measure as close as possible to the spot that you will focus through, below. Focus the microscope on the top surface spot, and measure the distance from the surface to any convenient spot on the microscope. I use a pair of calipers, but any accurate rule will do. Move the microscope down, and focus on the bottom spot. It will travel less than the real thickness of the glass.