If the sign says "Croissant au beurre", you can safely assume it is made with butter. A "croissant ordinaire" is made with margarine. However, I don't think I've ever seen the term "Croissant ordinaire" at a boulangerie. Sometimes I've seen just "croissant", and other times both "croissant" and "croissant a beurre." I've had a number of bad croissants in France, but I don't know if it was because the baker used margarine, or if he/she was just a bad boulanger. I don't think I've ever had a bad croissant in Paris, though (but several bad baguettes).