You can use a caliper. I use a small cheap plastic one most of the time. As for thickness some people like 1/8", but I usually make them just under 1/8 about 3mm. I guess it depends on the wood too cause some if its too thin can break easily. Burls for one if not stabilized tend to break. Tight grained, harder woods work easier thinner. You can test the wood and see if it flexes or bows to give tension when folding to see if it is thick enough. The wood doesn't have to flex alot. By test I mean put the razor into the scales and fold it not bend with your hands. That is if you have some small nuts and bolts.