Optimally, a micrometer for the openers and a caliper for the closers. Also, a closer measurement tool is absolutely necessary to get your closer shim thicknesses.
But that said, you can measure both the openers and closers with a micrometer. Its just that with a mic you have to go around the perimeter of the closer shim and its not quite as accurate as a caliper. But it will do in a pinch if you have to.
Now with a just a caliper and the closer measurement tool, you can zero out on the tool then use it inside the bottom of the opener shim and measure the thickness of the opener.
I have included a diagram showing the closer measurement tool in use.
www.mcmaster.com has micrometers and calipers for decent prices.
Hope I havent completely confused you!
Mike