The best way to determine a guitar's value is to start by researching a guitar appraisal website; sites typically offer rough estimates based on make, model and condition. After conducting online research, a guitar technician or historian is a smart choice to help further determine specific value.
Like any musical instrument, guitars are subject to many factors that determine their worth. It is important to get guitars checked out thoroughly, especially vintage models, because playing condition and appearance have significant impact on value. Experienced dealers and collectors understand that "vintage" is often simply a euphemism for "old." On the other hand, an aging guitar in rough shape that was played by Jimi Hendrix could be worth tens of thousands of dollars despite its condition. Major brand names, such as Gibson and Fender, tend to hold their value better than lesser-known brands.
Guitar appraisal websites are a good way for non-musicians to get basic feedback about their instrument's worth. Many of these sites charge a fee for their appraisal. It is important to visit a qualified guitar technician as well, because they are usually able to play test the guitar and give a more accurate assessment about anything that needs to be fixed or replaced.