Identifying used steel is not easy. You can use a spark test to compare it with known samples of steel:

Wrought Iron
Wrought iron sparks flow out in straight lines. The tails of the sparks widen out near the end, similar to a leaf.[1][5]

Mild Steel
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.

Medium-carbon steel
This steel has more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel.

High-carbon steel
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
Manganese steel has medium length sparks that fork twice before ending.

High-speed steel
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
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
Cemented carbide has sparks under 3 inches, which are dark-red in color and do not fork.

Titanium
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.

Agricultural Steel 1080

Axels 1040

Ball Bearing Balls 52100

Ball Bearing Races 52100

Band Saw Blades L6

Bits, Router M2

Bolts, Anchor 1040

Bolts, Heat Treated 2330

Bolts, Heavy Duty 4815

Brake Lever 1030

Cams A6, S7

Chisels O2, O6, L6

Clutch disk 1070

Clutch Springs 1060

Coil Springs, Auto 4063

Coil Springs, Truck 5160

Cold-Rolled Steel 1070

Connecting Rods 1040

Crankshafts 1045

Cutters, Bolt S2, S7

Drifts L6, S2, S7

Drills M2

End Mills M2

Fan Blades 1020

Files W2

Gear shift levers 1030

Gears, transmission 3115

Hammers L6

Harrow Disk 1080

Hay Rake Teeth 1095

Jackhammer Bits S5

Knives, machine M2

Leaf Springs 1085, 5160

Lock Washer 1060

Mauls L6, S2

Mover Knives 1085

Music Wire 1085

Nail Sets L6

Plow Beams 1070

Plow Disk 1080

Plow Shares 1080

Pneumatic Tools L6, A6, S7

Punches Cold A2, O2

Reamers M2, O2, A2

Roller Bearings 4815

Screw Drivers L6, S2

Snap Rings 1060

Spring Clips 1060

Spring steel, Clock 1095

Steering Arm Bolts 3130

Steering Arms 4042

Taps M2, O2

Transmission Shafts 4140

Tubing 1040

Universal Joints 1145

Valve Springs 1060

Wrenches L6, S2