The carbon is present in the form of iron carbide scattered amongst the iron molecules. The creation of either extreme requires, however, heating to the critical temperature first to form austenite. Workable properties are achieved by reheating the martensitic iron to an intermediate temperature which alters the crystalline structure and makes the metal softer with progressively higher temperatures. So much for theory. In practice you can buy the raw material with a known carbon content or test scrap material around the shop for carbon content and use what appears to be high carbon.
Shovel blades make excellent springs, for example.
High carbon steel touched on a grind stone will give a shower of fizzy, bursting sparks. I suggest trying this on a file as an example and compare it with what you know to be low carbon such as angle iron. After shaping the part to be hardened to its final form, polish it and heat it red hot. Plunge it into water until cool. For long round parts put them in a drill chuck and quench them spinning to prevent warping.