Zen Knitting

The most accurate way to measure the diameter (the width across a circle) is with the use of a micrometer. It is also used for measuring thickness of sheet metal.

But you can also get a hook gauge http://en.wikipedia.org/wiki/Hook_gauge that has a series of holes in it, and you insert the hook into one that accurately fits and you know the size that way. These are also used for knitting needles in the same way. These can be made of metal or plastic and cost from very little to a lot depending on how fancy they are. Check your LYS (local yarn store) for one.