To some extent, the answers will be dependent on model you are looking at, the amount and types of customization that have gone into the bike, and the amount and type of actual use it's experienced. If it's been to the track (probably not an issue with touring bikes or cruisers, but certainly possible with sportbikes), wear and tear on the drive train will have been much heavier. On the flip side, a bike that has only 1000 miles on it in 5 years could have problems with corrosion or bad fluids.
First and foremost, check for legal problems. Get the VIN number and have it checked (I believe there are free online services these days) to make sure the title is clear and not a salvage, i.e. it is not stolen, has no liens you don't know about, and was not wrecked. This may not be as big of an issue with Goldwings, but very important for newer motorcycles, sportbikes, and Harleys.
Second, you should consult http://www.recalls.gov/nhtsa.html to see if any recalls have been issued for the bike or its parts and that any repairs or replacements have been done. Sites like http://www.mcreports.com/ may also be helpful in researching the quirks of particular models.
Third, I would see if I could obtain the manufacturer-recommended maintenance schedule (from the owner's manual) for the model and year you're interested in. It will provide the official guidance as to when various parts and systems should be checked, adjusted, and/or replaced. You can then inquire with the current owner as to if and when such maintenance was performed.
These are all things to do before you even set eyes on the bike. For the actual meeting, you can consult checklists such as http://www.clarity.net/adam/buying-bike.html . That guide is particularly thorough, but it only takes a few seconds to check fluid levels and clarity (coolant, oil and brake, and on some bikes clutch/transmission), chain tension and cleanliness, tire health, and brake pad thickness; or to ask the owner about the last oil change, coolant change, or clutch cable adjustment. If something is amiss here, other more serious maintenance may have been neglected.