I think most of the 'standard' Case shields are probably 'nickel silver' (metal), as with the bolsters. However, there have been some deviations among special lines. Case used to make a 'red shield' series of knives which, I'm pretty sure, had molded or cast shields (painted red). I also have one or two 'Barb Wire' series Cases with cast & painted shields. See pics below for each. Not sure what the cast shields were made of, but I'd think some form of plastic was probably the least costly option. The nickel shields will usually patina or tarnish a bit, over time, so that might be one way to tell. And I bet the stampings/engravings in the nickel shields would displace some material around the perimeter of the stamps, which probably wouldn't be seen on the cast/molded shields (use a magnifier to look).
My Case 5344 SS 'Red Shield' stockman, alongside my Schrade 8OT:
My Case T.B. 6339 SS Tony Bose 'Barb Wire' Sowbelly: