Stuart,
IMO the best way is by using a proper wall thickness micrometer. Measurements should or need to accurately be to the ten thou (0.0001"). Like any precision instrument it needs practice and care to develop a consistent operating technique.
We have them in stock by Mitutoyo. The K&M guys and ourselves get these modified in batches by Mitutoyo where the anvil has a bevel ground so you can read neck wall thickness down to the shoulder junction if you want. There is a slight ball shape on the cross pin (out of view in this photo).
Not cheap unfortunately but we don't find any other way that is as good. Mitutoyo tools are top shelf. Buy once, own for life. I still use many Mitutoyo tools which I purchased in the 1970's.
http://brtshooterssupply.com.au/products/mitutoyo-wall-thickness-micrometre-0-1inch.html
Every shot pleases somebody..........