If you have one that looks similar to the one I posted a picture of, then be careful to find out what it says (in German) on the rubber, it will specify.
More than likely, if it doesn't have any notation you are OK to mount it either way. The rear one typically, but not always, goes on either way (solid rubber, no "flex" areas around the metal inserts).
The front flex disc will be noted as an inspection item if you look at the official inspection sheets ("check front flex disc for wear"). The rear one does fail, but it's not something you'll see noted on inspection sheets (not considered a wear item like the front one). The rear one is usually a lot more durable than the front.
But I do remember having a failed rear one (not much left of it) and the parts guy brought out what I felt was the incorrect rear coupler, looked just like the front flex disc, and after he insisted it was the right part number, I checked the EPC with him and sure enough, this car used what I would call a flex disc.
Gilly