Chamber headspace or cartridge headspace?
To measure chamber headspace, a set of headspace gauges is usually used to determine if the chamber is within GO/NO GO/FIELD dimensions.
I understand that, unless you are finish reaming a chamber in a new barrel (or one that had the barrel turned back a thread), there is little use for a GO gauge. Someone else has already reamed your rifle's chamber to a little over GO before they sold it.
The NO GO gauge is the useful one. If you have a rifle that does not close on a NO GO gauge, it has adequate headspace but not excessive headspace. If, after firing thousands of rounds, the stripped bolt closes on the NO GO gauge, you should consider having the rifle rebarreled.
The FIELD gauge is sometimes referred to as a FIELD REJECT gauge, because the idea was that armorers in the field would use it as a check to see if a rifle could be returned to service or sent in for a new barrel. Field armorers were not concerned about brass life, just whether the rifle would suffer case head separations with new military brass. Many believe that this gauge still can be used the same way, to operate a barrel beyond NO GO dimensions. Considering the original intent, and that today's shooter may be using reloaded ammo, the least that could be expected is greatly reduced brass life, with the occasional case head separation not surprising.
Shims of known thickness can be stuck to the bolt end of a GO gauge with a film of grease to try to determine what actual headspace is for a given rifle.
Cartidge headspace, on the other hand, is usually measured with something like a Wilson cartridge headspace gauge. There are shoulders for minimum and maximum permissable headspace. A shooter generally adjusts his resizing die to fall between these two steps, which are usually on the order of 0.006" apart. Semi-auto rifles may require minimum cartridge headspace for reliable chambering, and bolt-actions typically do not. For bolt-actions, a cartridge headspace 0.002" less than the fired case length to datum (in that rifle) is usually OK and results in increased brass life (due to reduced stretching.)
There are far more sophisticated tools and techniques available than I have described above. However, I believe the above is generally correct and OK as an introduction to the two types of headspace.