A combination of the Blue book and the Vintage Guitar price guide gives you a good feel for most models. To get a good idea for what popular examples are selling for, search, under advanced search, in ebay for completed listings. An average of actual sold examples will tell you what the market is, if you have enough examples. At the end of the day, it comes down to if the instrument is speaking to you. Buying without playing has it's risk, but out of 20 (ok, I said the number
) plus Guilds I've bought in the last few years, only a couple didn't work for me - and the people I sold them to love them - so maybe it's me!
All I can tell you is look hard, but don't cheap yourself out. The one that got away from me was a DV 62 that I fretted over for a week and then tried to get 50 bucks off - while I talked to one guy in the shop about it, on the phone with my credit card in my hand, another guy sold it to a fellow for their asking price, which was $925! I love deals, but I love nice guitars even better.
Good luck!
tj