Lift up the boot carpet and you should see the sticker to the left of the spare wheel well, stuck to the floor pan - if its not there I would suspect possible accident damage and repair. The same sticker is also stuck inside the cover of the service record book.
If its a GT spec car, the sticker should state 'GOLF TDI HIGHL' for highline spec. In the next line, the first figure is the engine power in kW, for instance my 130 TDI is listed on the sticker as 96.