Colt offered bright nickel that looks like a mirror, and a satin electroless nickel finish they called "Coltguard".
This has a satin finish that does not have the brilliant mirror finish of bright nickel.
If you compare most any nickel plated gun, whether bright nickel or electroless nickel to stainless steel, the nickel has the distinctive yellow tint of nickel.
Stainless steel has a "white" color.
Simply compare your Mark III to something you know is stainless steel. A kitchen utensil will do.
If your Colt Mark III is a 1972 model, it's probably bright nickel, which means it'll be shiny as a mirror.