For decades I’ve used cheap plastic bolt checkers, with plain holes to confirm diameter and simple scales to measure thread pitch.

This is way, way better. It’s all-metal, and has actual bolt studs and threaded holes to instantly identify nearly any metric or SAE fastener. It took me seconds to identify a couple of weird bolt sizes (3/8 x 24, 1/4 x 28) on my 1976 Triumph TR6, which could theoretically have a mixture of SAE and metric fasteners.

It has holes at each corner, so if you wish you could mount it on the wall behind your workbench.

Made by Hillman. I found it at an Ace Hardware for $30, but you can probably get it or a similar model on Amazon. Highly recommended.