You can use your enlarger, if you have one, to measure the diameter too. Put a clear plastic ruler in the carrier, measure ( for example ) how many mm a projected 2cm is, and then a little math to figure the diameter of the pinhole. I've only gone to this trouble twice. Usually I just put the pinhole next to a ruler with mm markings and look through a magnifying glass. You can measure to within a few tenths of a mm that way. My favorite pinhole camera was made like this.. it's something between .25 and .3mm and it doesn't matter a bit what it actually is because it makes great pictures.

Another trick is to hold the pinhole up very close to your eye and look at something bright. You will easily be able to tell if the pinhole is round and if there are any substantial burrs.