If you work with metal frequently, you can probably tell what gauge material you’re working with just by sight and feel — for the rest of us, Starrett makes a precision US Standard Gauge for measuring the thickness of uncoated sheet, plate iron, and steel.
Starret makes this gauge in the USA, and though they don’t say what material they use, we’re betting steel. They give it a satin finish and mark one side with US standard gauge and the other with the decimal equivalents. After hardening the gauge, they carefully test it for accuracy.
If it’s not too late, you can purchase Starrett’s model 283 standard gauge for $24 at Amazon, which would make this one of Starrett’s more affordable tools. Normally they’re priced around $50.