AFAIK, they have one (1) of their long-haul planes set aside for flights to Continental Europe. It used to be an A340, but these days it's usually an A330. Which route they put it on depends on demand, FRA has quite good chances, but e.g. on weekends at the beginning and end of school holidays we also regularly get it to ZRH.

You should be able to tell from EF by pulling up a seat map, perhaps around a week before you travel.