IMO, the best way.. have it written in the sales contract that *no* prep, not even checking fluid levels or removing shipping plastic card board, plates, etc. is to be done; you want it "in the wrapper". If your car didn't have plastic on the seats, they did do at least *some* prep.
Tell the sales manager that you want to be called as far in advance as possible as to the exact time the car will arrive, and another call before the delivery truck arrives.
Get to the dealership before they take it off the truck. Have them pull it off the truck and into a parking spot you can keep an eye on while you finish the paper work. Yup, that's pretty anal, but so am I
You'd be surprised at how much protective stuff there is in an un-prepped new car. Usually, dealer prep guys have to go through a prep checklist that they sign and hand to the sales manager. It's not just cosmetic; they check vital fluids and other important things during "prep".