I've written about this (Seven Tips For Buying A Used Acoustic Guitar - ) before but that was a short and simple checklist aimed at acoustic guitars. This is more detailed and can be applied to acoustic, electric, regular, baritone or bass guitar. My goal is to. asz help you overcome your emotional desire to buy that beautiful (whatever) brand and model and force you to use some real due diligence to make sure you want what you are buying.
While I am not claiming this list covers every possible thing you should look for, I AM claiming it gets pretty close. Good luck.
1. Does it make sense?
a. Start by using the "does it make sense" test. If someone tells you a guitar is brand new yet it smells, or the case is very old or the strings are rusty, then you know you have a problem. Before even looking at the rest of this list, just move on. There are plenty of guitars out there you can buy. What you DO NOT want to do is buy a problem.
2. Go over the headstock
a. Tuners work and aren't sticky
b. Tuner shafts aren't bent or crooked
c. Note cracks, paint runs, scratches, de-lamination
3. Go over the nut
a. What kind of material (bone, plastic, tusq?)
b. Is it the correct height?
c. Is it properly seated - and is it flush?
d. Do the strings bind in the nut?
4. Do a general cosmetic checkup
a. Judge finish quality
b. Check fretboard surface
c. Check top, bottom and both sides of the instrument
5. Check guitar construction
a. Check for loose parts of any kind such as inlays
b. Check out the neck to see if it is straight
c. Check out the neck to see if it is properly attached to the body
d. Check to make sure the bridge is solid
6. Misc hardware checkup
a. Make sure strap buttons (if any) are solid
b. Check the condition of the truss rod cover and truss rod
c. Inspect the pickguard
d. Test output jack
e. Test switches and potentiometers
f. Check for loose pickups
g. Check string trees if any
h. Check battery if any
7. Misc electronics checkup (if applicable)
a. Check the snugness of the output jack and make sure it works
b. Make sure the pickups work and that the switching between them works
c. Check onboard tuner if any
d. Check pre-amp if any
e. Check onboard effects if any
8. Playability checkup
a. Check that the guitar holds tune
b. Check the neck relief
c. Check for sharp edges on the frets
d. Check action height
e. Check intonation
f. Check tremolo if any
This is a fairly detailed checklist. By bringing it with you when you shop for a used guitar you will greatly enhance your chances of getting the instrument you wanted at a fair price.