If you don't want to forge it down to test and have the ability to cut a slice off, I'd go that route. Some people call them "coupons" and do testing with their steel that way. You can have a thin cross section and not waste forging it. Then heat up and quench like you normally would and see what you get. Since its mystery steel, if it ends up hardening nicely, id cut several coupons. Then you could harden them and try different tempering cycles and see where each ends up to give you a good idea moving forward with a knife.
Jeremy