This is what you can do ... The model of the headlight might be on the discharge tube itself. I had the problem with my vehicle where I ordered what I thought were the right replacement bulbs for the fog lamps (H7) and found them to be the wrong ones. I went and bought another set (H11) and they were wrong as well. I took the lamp out and discovered they were actually H8 bulbs as it was printed right on the bulb base. Anyway, it could be printed on yours as well.
Second thing (which might actually be the first place to look) is the owners manual. They might have the model listed there.
Looking online at the difference between the two, they are physically different. The D1S has a square metallic base with the connection going out the side of it. The D2S has a round base with the connection at the end (bottom). Here are some pictures for your viewing pleasure:
From the looks of things, you want the D2S, but make sure before you buy. Remember, you want to replace these in pairs so the color will be the same. Also, when replacing, wear clean gloves (like nitrile or cotton if you can find them). If you get oil from you fingers on the bulb (or any other contaminant, you can kiss those bulbs goodbye. Their life expectancy will be GREATLY diminished.