How to recognize Keramag / Geberit KeraTect?

Right, you're right

And don't be upset about the few euros, we also bought it a few years ago

Should in any case be on a side visible even after assembly.

After installation, it is no longer visible. It is located on the back, which is against the wall. On the side, there is a sticker that says KeraTect, if it has not been removed by the installer.

After installation it is no longer visible. It is located on the back side, which is against the wall.
On the side there is a sticker that says KeraTect, if it has not been removed by the installer.

Says who? We also have a Keramag toilet with KeraTec coating (ok, I see no real difference compared to one without or a Villeroy & Boch, which we also still have) and there it is clearly visible on the side. In my opinion, that is also not a sticker! Did the installer/seller say that now?