Almost all the non-GT4 or GT-5 cars sold in Europe were called 'GTS' so it really wasn't a big deal there, just another model. Outside of the original bill of sale or contacting the factory for their records, I know no way to verify this on an individual car, after all this time and the possiblity of adding parts by owners. Almost all Euro cars would have all-Itaian instruments, a roof radio antenna and might be missing a couple of Us added chassis braces.