This comment confuses me. If you don't know if something is stainless (versus zinc coated), using a magnet will not necessarily help. Most stainless steel that you will encounter (outside of certain industries) is NOT magnetic. Magnetism is not an indication of quality of stainless steel. It is an indication of the additives.
Ferritic and martensitic stainless steels are magnetic. Austenitic stainless steels are non-magnetic. Ferretic is cheaper, but is less corrosion resistant, due to lower chromium and nickel content. Most compositions include molybdenum; some, aluminium or titanium. Martensitic is highly machinable, but more brittle. Martensitic stainless steel contains chromium (12–14%), molybdenum (0.2–1%), nickel (less than 2%), and carbon (about 0.1–1%) (giving it more hardness but making the material a bit more brittle).
The vast majority of stainless steel bolts/nuts are 18/8 austenitic, which means they contain 18% chromium and 8% nickel. Not magnetic.
ALL zinc-coated steel IS magnetic, as is nickel-coated steel (which is considered bird-safe). Uncoated steel, "carbon steel", does not have a higher level of chromium like stainless does. The chromium in stainless steel forms a chromium oxide on the surface, which limits oxidation of the iron (rust).
Also, "food grade stainless" is not an official (regulated) term, so you don't necessarily know what that means. In practice, it can mean either non-magnetic austenitic stainless (300 series which includes 18/8, 18/10) or magnetic ferritic stainless (400 series).
(The percentages and fancy words are taken from wikipedia's article on stainless steel since my memory is weak...)
I'm sorry for the diversion from the subject, but don't assume that magnetic means safer - it typically means it ISN'T stainless.