I use this handy device constantly to check and compare thickness on a variety of projects. Slide the gauge over the edge of the stock, keeping the graduated edge flat on the surface of the board, and read the thickness. You'll have to make the graduations yourself, based on a board you gradually plane thinner and thinner. If you want to record some odd size, make a pencil mark on the lower edge. The more gradual the taper of the opening, the more accurate the gauge.
This is really cool, because it essentially magnifies the dimension for you. For example, if your taper is 1:8, every horizontal inch is only 1/8" of thickness, and 1/2" would represent 1/16" of thickness, etc. You could make the graduations as recommended above, but if you know the taper – and it's accurate – you can mark the graduations using a ruler.