When I almost bought a 530 at auction I traced the entire service history in an afternoon despite not owning the car. Here is how I did it, incase its useful to anyone else.
Firstly, call BMW Customer Information. The phone number is on the BMW website. Explain you are buying a used BMW and wish to check the service history is genuine. They will then provide you with a list of Dealership Visits.
This list simply contains the date a car visited a dealer, which dealer it visted, and the phone number. Thats it. Nothing more.
You then phone each dealer and repeat the story. Some can be funny about it, but most will help - if it's had a straightforward service they can and will tell you over the phone what it was and the mileage of the car. They'll also tell you if its had repair work but if it was paid for by the original owner they won't tell you any more.
I built up a comprehensive service history - from day 1 to present day - of a 135k mile Y plate 530i. Which sold for stupid money and then appeared on Autotrader 2 weeks later with just 70k miles on the clock but with an 'unfortunately mislaid' service book. People DO clock BMW's, so do your best to avoid one.