If your guitar sounds in tune when playing one chord, and out of tune when playing another, then your intonation is probably out of whack. Learn how to check and correct the problem here.
This article takes a look at the saddle of the guitar, and how it affects string breakage. If you're a guitarist who tends to break a lot of strings, this may answer some questions for you.
If you've got a crack in your guitar, all is not lost. It could simply be a crack in the finish. This article details how to tell the difference between different sort of cracks.
If your frets are too low, it can affect intonation, and cause many other problems. This in-depth article explains the (complex) process of checking fret height.
A poorly carved nut can be the cause of many difficulties - tuning problems, fret buzzing, poor instrument tone, and much more. Learn all about what makes a nut good, or not so good here.
Sometimes, high string height can be the result of a guitar's tendency to slightly change shape over time. This results in a change in neck angle. If you've got an accurate ruler, you should be able to perform this test.
Have you ever seen guys hold a guitar, and look "up the neck" of the instrument? This page will describe how to do that (it also contains a bunch of other stuff, but ignore that for now).
Lightly tapping your guitar can help you identify certain problems the instrument may have (loose top, braces, etc). Get the scoop on how to perform a rap test here.