What other types of nuts will you have in your inventory?  If you only use coated steel nuts (zinc plating, organic flake coatings like Magni, etc.) then the Nitronic stainless steel nut will appear different-- no coating.  Are the Nitronic nuts going to be machined/turned out of bar, or are they going to be cold formed out of wire?  The machined version will probably have a bright surface, but the formed parts will probably be duller, unless they go through a passivation treatment prior to shipping.  Your vendor of these parts should be able to assist you with proper identification of these nuts versus nuts made out of other materials/coatings.

Pucp,
If I am not mistaken don't these bolts have a unique marking on them?  As a inspector that is always how I tell.
Like TVP said the vendor who is supplying these should be able to give you specific markings. Hope this helps

Don't forget the obvious; they will be non-magnetic. Also the density will be several percent greater than that of their carbon or alloy steel counterparts.