Set of 6 Hardness Tester Checker File 7.1 inch HRC40-HRC65 Hardness Tester File Set kit Suitable for Industrial Applications Rough Estimation of Metal Rigidity with an Oxford cloth Storage Bag

You can get some from Amazon: https://www.amazon.com/TSUBOSAN-Japan-Hardness-Tester-Checker-HRC40-HRC65/dp/B000TGHFLC/ Looks like cheaper brands appear to be OK too, judging by the reviews: https://www.amazon.com/Leriton-HRC40-HRC65-Industrial-Applications-Estimation/dp/B0FT86K6JP/ You don't need a full set to see if steel is hardened (that to get a good idea of the actual hardness). Just to check if it's hardened, you only need the softest one (40HRC and up = hardened). You might be able to buy a single 40HRC file for less.