Once upon a time I saw a set of calipers that read out wire size. I frequently get call to remove old subpanels and need to check the size of wire an the panel that the load is being moved to. Most of these the marking on the wire is no longer legible. If anyone can help me I would appreciate it. Also this board is a great asset and very interesting.
The smaller sizes can be checked with a pair of strippers that have the multiple sizes. For the larger sizes, you might check the dimensions of legibly marked conductors, log them in on a piece of paper, and use them for future reference.
I usually use strippers to determine the size like electure mentioned and carry around some small pieces of larger sizes to compare side by side. If it's important for larger sizes I think you can figure it out by knowing the size of each strand and counting them.
SPKJPR, I BORROWED TWO OF THESE METAL WIRE GAUGES , BOUT 3" IN DIA. WHEN I WAS IN THE NAVY AS A SPARKY. FUNNY THING IS I DON'T RECALL GIVING THEM BACK. I BET THERE WAS 20 SIZES ON IT. I THINK MIKE HAS THE BEST ANSWER.