If the standard car had the GTI logos then short of pulling the engine out to get a look at the diff, or getting a wheel off and measuring the diameter of the disc brakes, how would you know if the car had the PP on it or not?
I do wonder how much problems there might be buying second hand as it is... Although the same issue would occur with the DCC. I've never bought a used car that claimed to be fitted with options that would not be visible, what normally happens in these circumstances? Can you check the car's spec on a database somehow?