There are resources that can help, like Vintage Guitar Price Guides. You can also subscribe to Vintage Guitar Magazine at vintageguitar.com  Find an online forum for the brand of guitar you are considering buying and see what you can learn there.

Be aware of the possibility that the local seller you have in mind might not have a vintage guitar at all. Vintages can be hard to come across, 'cause if they weren't then they wouldn't be so valuable any more, would they? But there are lots of places to expand your search as to whether a vintage guitar has been put up for sale. Check newspaper ads in addition to the internet. Nowadays you can find out about a vintage guitar that is up for sale even if it is in the next state or even within the entire country.

The thing that you must be careful of when ordering a guitar over the Internet or from a distance is never to pay the full amount in advance. Promises are made to be broken, and a vintage guitar may not turn out to have the advertised quality when it eventually arrives at your doorstep. Always use a post-paid mailing service. That way, you don't pay until the instrument actually arrives. While not totally fool-proof, it's one way of ensuring a minimum quality.

The best thing to do would be to go to the person or store that is selling the guitar. Try it for yourself. Make sure everything works and the condition is as advertised. For a large investment, you may want to have it appraised. There are professional agencies that appraise guitars as well as other instruments for a fee.

Understand that a shop will sometimes ask a hugely inflated price. If it is a consignment shop, the actual owner may not even get to know the price at which the instrument actually changed hands.