Some tips to test the hardness of the steel in the bolts that we buy:

Copied from:
http://www.metalwebnews.com/machine-tools/ch2.pdf

Spark Test

This is a simple identification test used to observe the color, spacing, and quantity of sparks produced by grinding. It is a fast and convenient method of sorting mixed steels with known spark characteristics. This test is best conducted by holding the steel stationary and touching a high-speed portable grinder to the steel with sufficient pressure to throw a spark stream about 12 inches long. These spark patterns provide general information about the type of steel, cast iron, or alloy steel. In all cases, it is best to use standard samples of metal when comparing their sparks with that of the test sample.

File Test
One simple way to check for hardness in a piece of metal is to file a small portion of it. If it is soft enough to be machined with regular tooling, the file will cut it. If it is too hard to machine, the file will not cut it. This method will indicate whether the material being tested is softer or harder than the file, but it will not tell exactly how soft or hard it is. The file can also be used to determine the harder of two pieces of metal; the file will cut the softer metal faster and easier. The file method should only be used in situations when the exact hardness is not required. This test has the added advantage of needing very little in the way of time, equipment, and experience.

Some pictures of spark tests:

http://wearanswers.com/Instant-Answe...ification.html

Mild Steel
Identification: If it looks like steel, weighs like steel, but is magnetic, you're down to whether you have mild steel or a harder steel. On the spark test look for longer streamers from the wheel with fewer sparkles.

Hard Steels
Identification: If it looks like steel, weighs like steel, but is magnetic, you're down to whether you have mild steel or a harder steel. On the spark test, the sparks will tend to end closer to the wheel, and the more carbon (i.e. the harder the material), the more sparkles you'll see. A noticeably different spark, perhaps with unusual colors and shapes implies an alloy steel of some kind.