A very good way to tune a 6-string guitar by ear is by using harmonics. The physics of guitar harmonics is much too complicated for the scope of this article, but here is a quick explanation as to what harmonics are. When you pluck a guitar string you create a variety of vibrations and frequencies that our ear translate into a specific note. This is called the ‘fundamental’ note. Harmonics are created when you, in essence, break the sound wave in halves or quarters, creating a new tone. Harmonics are created by lightly placing your finger at a specific fret then plucking the string, creating a ringing sound. It takes a little practice to hit the sweet spot and get a nice sustained ringing, but it helps to place your finger directly over the fret and quickly remove your finger when you pluck the string.   

Using harmonics to tune your guitar allows you to better hear the difference between two oscillating sound waves and recognize when you get them exactly matched. This is a great technique to use when tuning by ear. Two different tones will produce and audible “wah-wah” sound. The shorter the interval between the “wah’s” the farther out of tune your strings are. Turn the tuning key slowly while listening to the wah-wahs. If the wave gets slower, your going the right way. Turn the key until you can no longer hear the oscillation. When you pluck the two strings together and don’t hear the wave, your strings are perfectly tuned to each other.

To use harmonics for tuning your guitar by ear, you will use the 5th and 7th frets to create the harmonics. Start with the low E string. You will tune all the other strings based on this string. Guitar strings are tuned to the notes E-A-D-G-B-E.

In this diagram each color represents a note. 

Blue = High ETuning diagram
Violet = B
Yellow = D
Green = A
Red = Low E

Pluck the two strings nearly at the same time so it is easier to compare the two tones.

To create the note for the 5th string, the A string, create the harmonic note by lightly placing your finger on the 6th string, at the 5th fret, and pluck to create the harmonic A note. To the same on the next string, pluck the 5th string at the 7th fret to create the same harmonic A note. You will want these two tones to be equal. Use the technique with the oscillating tones or “wah-wah” sound and turn your tuning key until your 5th string matches the 6th string.

Repeat this sequence for the rest of the strings with the exception of the 3rd string. This string will have to be plucked in the normal fashion because of the fact that it must be played on the 4th fret. A clear harmonic tone is more difficult to accomplish on this fret and you must use a different technique, so it’s easier to just use the normal tone. 

Playing harmonics takes a little practice but once mastered it is an excellent method to use when tuning your guitar by ear.