Hi there,
I am a bladesmith and the best way to test for high or low carbon content in the field is to heat the end of a piece to nonmagnetic, quench it in oil or water and then hit it with a hammer or hard rock. High carbon steel will break off easily and have very small grain ( as in to small to see ). Spring steel is most often in the 60 carbon series. I would guess that it might be 5160. One thing if it is an old spring steel from a car spring it may well have a bunch of micro cracks that will show up when you try to forge a knife out of it.