Identifying used steel is not easy. You can use a spark test to compare it with known samples of steel:
Wrought iron sparks flow out in straight lines. The tails of the sparks widen out near the end, similar to a leaf.
Mild steel sparks are similar to wrought iron's, except they will have tiny forks and their lengths will vary more. The sparks will be white in color.
This steel has more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel.
High-carbon steel has a bushy spark pattern (lots of forking) that starts at the grinding wheel. The sparks are not as bright as the medium-carbon steel ones.
Manganese steel has medium length sparks that fork twice before ending.
High-speed steel has a faint red spark that sparks at the tip.
300-series stainless steel
These sparks are not so dense as the carbon steel sparks, do not fork, and are orange to straw in color.
310-series stainless steel
These sparks are much shorter and thinner than the 300-series sparks. They are red to orange in color and do not fork.
400-series stainless steel
400-series sparks are similar to 300-series sparks, but are slightly longer and have forks at the ends of the sparks.
Cast iron has very short sparks that begin at the grinding wheel.
Nickel and cobalt high-temperature alloys
These sparks are thin and very short, they are dark-red in color, and do not fork.
Cemented carbide has sparks under 3 inches, which are dark-red in color and do not fork.
Although titanium is a non-ferrous metal, it gives off a great deal of sparks. These sparks are easily distinguishable from ferrous metals, as they are a very brilliant, blinding, white color.
You can also try quenching in various ways to see if the sample can be hardened. Trial and error will show you whether the metal you have is useful or not.
The following list identifies what the steel could be, but due to the huge variety of steel there can be no guarentees.
Read more: Recycled Steel identification