You are essentially measuring the diameter of the blank at the area you want to mount the reel seat.
A dial caliper is probably the easiest too to use to measure the diameter, but will set you back about $20. Most can also be used to measure inside diameters also, such as the bore of a reel seat.
A drill gauge, which is a metal or plastic sheet with a ton of different sized holes drilled in it can also be used. Insert the blank into the holes until you find the size that closest matches your rod. Not as precise, but good enough to help in selecting a reel seat.
With that in mind, I don't sweat small, or even moderate differences in blank diameter and seat bore. Bores that are too big can be shimmed solidly with fiberglass dry wall tape. Masking tape also works well enough with small differences between seat and blank.
A rat tail file makes short work of a seat that is bored too small. You can usually get a fit that requires no shimming.
To sum it up, I don't worry too much about bore size, I pick a seat that I like and is suited to my purposes, knowing that I can make it fit.
Kev