Basically, by the rust. If there is rust, it's "normal" steel. If no rust and you still wonder, try this: Use grinder on a piece of steel to rough it up and make sure it's not chrome-plated; then get the piece of metal wet. Let sit over night. If there is any discoloration, it's in the process of forming rust. As an added factor, before leaving it sit overnight, protect a section from the air/moisture by putting scotch tape on it. Next morning, pull off the scotch tape and look for any difference in the protected area.
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