Well if your guitar was built in the last 30 years
or so, you can probably figure out its worth from a local guitar shop with no problem.
However, if you own, inherit, or discover a vintage instrument, it may prove to be
more of a challenge to get an accurate appraisal. The net wisdom seems to be to give
a call to Elderly Instruments, Gruhn Guitar, or Mandolin Brothers, and ask them. Elderly has an
On-line evaluation form you
can fill out and they'll send you an estimate (at least what they'd be willing to pay for
it) via e-mail. From a serial number and general description they can give you
a pretty good idea of its worth. I believe some of them will give a certified
appraisal if you send them pictures. All of these places traffic large numbers of
used/vintage instruments. The monetary value of old/vintage instruments is directly
related to their desirability to collectors. These folks have their collective
fingers on this pulse.