Actually, yes, many dentists prep without looking directly into the floor of the prep. Don't forget, even if there was no water spray and a good angle on the mirror, chances are that at times you will not see everything cause the head of the handpiece will block your view. After you cut a few hundred, or maybe even before that, you'll get the feel for cutting the teeth and can tell from just seeing the part of the bur that is visible just how deep into the prep you are and how much axial depth you've created.
Until then, it is cut and check. Sometimes you will have a perfect view, many times you won't. So use the landmarks to help you and cut a little bit at a time and then check. If you know the length of the cutting edge of your bur is 1.5 mm or 2 mm, use it as a guide. There is a reason in dental school you only do a couple fillings in a session when you first start, cause you need to learn the tactile sense, and you don't get that right away.