It's a bit simplistic but the old rule was "if it rusts it's OK" is a good starting point.
Note that as well as plated/coated steel; older paints, particularly on steel, tend to contain quite a bit of lead that vapourizing in the forge and breathing is *not* a good idea.
The other one you might run into is a Be Bronze. Be can be very toxic so if you are unsure on a piece of brass/bronze---like it was an old tool---I would skip using it for anything but a tool.
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Thomas
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