The Blue Book of Guitar Values website is one of the most prominent sources for determining the value of a guitar; other similar services include May Music Studio and Gruhn Guitars, Inc. The major factors that determine the value of an instrument include finish, cosmetics, the presence of cracks, and modifications such as parts replacement. The Blue Book of Guitar Values rates a guitar as 100 Percent New, Excellent, Average, Below Average or Poor based on these factors.
The Blue Book of Guitar Values performs guitar appraisals and evaluations for a fee.
You can also get a good range for the value of your instrument by visiting online stores such as eBay or Gbase. EBay is the largest online market for musical instruments, and following an auction can help you identify how much people are willing to pay for a similar type of guitar. Gbase is a resource for the higher end of the guitar retail market, but their prices can provide a useful starting point.
If you prefer to have your guitar valued for free, there are resources for that as well including Prepal, Elderly Instruments, Plucked Out and Just Answer. Prepal claims to be the world's biggest online listing of used musical instrument prices, while Elderly Instruments offers free valuations for those looking to trade or sell their guitars.