Before I got the gauge rod set,I used a starett small hole gauge to measure.
I recommend a headspace go guage for accuracy. If you don't have either the small hole gauge or rod set, you could use nice drill rear shanks to get one tight and then use a micrometer to measure. Start with a loose one and wiggle front and back to tighten up the fit, then pick a snug fitting bit. Hopefully you have a full set of drills, letter, number, decimal, and maybe metric to work from. A small hole gauge works pretty good, but the guage rod set is tighter and much faster.