Service book should be stamped to prove servicing. Now depending if this is on long life servicing or not will depend on the amount of stamps. Longlife is 2 years 20k if memory serves correctly whereas the other option Is once a year at 12 or 10k. I'm going from memory though.
Also you'll want to see if there's any proof of the gearbox oil being changed, especially if it's an auto. This goes for the quattro system too.
Do all electrical items work?
Any proof of Cambelt and water pump change?
Or any evidence in service history of the egr or dpf being changed. As these 2 are expensive if they go wrong