It's easy to spark-test, then quench-test to figure out if a steel will harden sufficiently.
Make an educated guess about the steel. Once you've made a test blade out of it, and quenched it at what you guess the correct temp range to be, (start with a little over non-magnetic) start tempering it at the lower end of the spectrum (400) and flex test the edge at different hardnesses. With your edge sharpened at a useful thickness (0.020 or so) you should be able to flex the edge over a rod, but the edge should spring back when you let pressure off, not stay bent. When you find a temper range that allows this, do some cut testing. When you find that you can make an acceptable number of cuts in rope or cardboard and retain an edge, (cut test it against a blade of a known steel) and when you find that you can at least lightly chop hardwood without chipping the edge, you have an acceptable knife steel.
If the steel does not harden properly, you may need to use a slower or faster oil, and/or adjust your hardening temperature. It could take a while to zero in on what your steel wants. It can take a bit even if you know what the steel is to begin with.
I've made quite a few knives from older coil spring. Mostly I treated them as 5160 with OK results, but had a spring that acted more like 9260, and have some heavy leaves that may be 6150. The only reason I'll use a spring now is to make a knife for a gearhead, I have some Mustang springs for that.
