the best way would be to use an optical comparator
a good end mill should only be minus .0005 from from the specified size
when i make a part I'll measure the part a few passes before the finish pass and make adjustments from there.
some times I'll make test cuts in a scrap piece of stock and use that to make offsets or change the tool dims in the cam , works well when adjusting for a ball end mill
sometimes i get spoiled and use our machine with the laser setter :lol: