Genuine ones will have 2 screw holes on the inside, stick on ones wont.
Proceedure for fitting genuine ones is as follows:
Pull off the front part of the hoop cover, this is held on only with clips, so will yield with a firm tug.
When the fronts are remove, remove 2 x cross head screws and slide the rear part of the cover upwards.
Refitting is the opposite, although you will have to slide the screw hole over a metal tab, so a small amount of patience is required.
The front part just clips back in.