Medicare has a great website: http://www.medicare.gov/Coverage/Home.asp
You can key in your id# or state and choose what kind of benefit you want to review. Your ID is better because it will include the benefit level you are paying for (Part A and B).
I use this all the time. The main site also has a listing of covered preventive care item.
The Answer you go is correct in getting the doctor to submit it prior to doing the test. My mom had an MRI given by a family doctor, he referred her to a specialist, and the specialist ordered a second one without even looking at what was sent by the first doctor. If I hadn't said something, she would probably have been stuck with paying for it since there was no reason for the second one. The first one was all the second doctor needed to know.
Follow the answer given and check out the website.