Check out Kelley's Blue Book, then take into consideration any "Extras" that the current owner has added, and the condition it is in. I like to ask questions like "Why is he selling it?", "Service records", in other words, find out if it's a piece of junk. The age of the owner shows a lot. How many owners shows a lot, was he the original owner?. If the owner bought it, thrashed it, and now wants to sell it, forget about it. If your lucky, the owner is a weekend warrior that maybe rode the bike 3 times a year. You have to be a detective and look for signs of abuse and use Kelley's Blue Book as "Reference".
http://kbb.com/