Hi,
There is now way of telling if the ceramic washers have been replaced apart from removing the supercharger and checking, the dealer that serviced the machine will have a record (maybe) if not the person you bought the ski from will have the workshop report and reciept (maybe again).
I know people who just get the torque rechecked before selling as the supercharger kit cost 300 pounds.
My advice to you is slip on the boiler suit and remove the supercharger, if you are mechanically minded it wont take more than an hour to remove, this is definitely worth doing as I have seen the damage that the ceramics do to the engines, as soon as the ski cant get top end rpm, have it checked.
Hope this helps
Mo.