Mechanically by a fixed reference. One turn of the barrel equals 25 thousandths of an inch- sometimes called 25 mills ( do not confuse with milimeters) therfore it can be read-off by an experienced operator.
I believe you are talking about dies and taps- which have a sort of inward cut Maltese Cross appearance, or cloverlead appearance in the stock, which is placed in a holder- and the job begun.
You unscrew them( remember that the left one has a left-hand thread) and measure the diameter of the threaded end with a vernier caliper. Should be either 9/16"(14.3 mm) or 1/2" (12.7 mm)