Only the basics
Apart from all the basic stuff like service records, smoking engine, excessive or unusual noise (good luck with that one), brake smoothness, rideability etc, the only other worthwhile check is the rear wheel play at 12/6 and 3/9 positions. Any bike that rides fine can have issues about to rear their head. I doubt the dealer will allow you to drop rear drive oils, or check splines, or do any other sort of tear-down. Second hand? Good condition? Reputable dealer? You still take your chances unless you get an iron-clad warrenty period that covers all mechanical failures in the first x months or x miles. That is something we don't get here with second-hand motorcycles.
Best thing you can do is go through the existing records and confirm any major issues in the past. If the records are just book stamps on service intervals, or no records, then it had better be cheap enough to cover any realistic potential failure. Can you contact the last owner? That's always a good thing to do here, if the records identify them and the dealer is willing to put you in contact.
2002 R1150GS Adventure
IBA # 39193