The way you measure any thread.
• First measure the diameter. It would normally be about 0.002/0.003" smaller than the nominal. So for a 1/2" thread, expect to read something on the order of 0.498". On the other hand, if you see something like 0.470", then suspect M12 (aka 12mm).
• Then measure the "pitch", or the number of threads per unit. For Unified threads, that unit is the inch. Pitch is commonly measured with a "thread pitch gauge", but you can do the same thing with a magnifying glass and a good scale. It helps to know where you are going. 1/2" threads are commonly available in 13 and 20 TPI (threads per inch).
So let's say your caliper measures a thread at 0.497" diameter and you have counted about 9.7 threads within a distance of 1/2 inch. There's no such thing as a 1/2" thread with 19.4 TPI, so common sense says it must be: 1/2-20 UNF.
