Dealer stamps are complete BS......unless you have an itemised account of work carried out.
Any stamp is worthless unless verified work is logged.
I can take one of my RRs to a dealer and tell them all I want done is the oil and filter and you will still get a stamp.
Best to look out for things like plenum filters being replaced, cleanliness under the hood. Tires, pull a diff, transfer case plug.
Get a good look at the coolant hoses......have they been changed if the RR is more than say 8-10yrs check the ones you cant see at the back of the engine.
Drive belts that are cracked/worn on a RR that is more than 6yrs is a sure sign that it hasnt had a service (properly) in a long time.
If you cant do this get an inspection done by some one competent.
I wouldnt buy any RR that hasnt had the trans fluid changed if it has more than 100,000km on it, If the trans fluid has been changed more likely than not the owner gives a about his RR.
Here in Aussie a full service will set you back at least $1000, people cut corners, when it stops or breaks down they say that the RR is unreliable, service it well and it will most likely be as reliable as a JAP car.
RRs are expensive to run, I expect to spend at least $1000 a year to own one and I do all my own work!, some years I get away with just the filters and the oils.