Playing guitar by ear is the capacity that some musicians develop to learn an entire song without looking at scores, or even a guitar tab. It is not an innate capacity, but something that musicians have cultivated for a long time.
Although it seems almost too complicated to play by ear, many people have found that developing this ability is not so complicated after all. If just takes some dedication and practice.
Basic Strategies to Play Guitar By Ear
You can follow some simple strategies to achieve this. For example, according to Corey Lewis, the basic steps are as follows: “Once you have tuned your guitar, you should figure out the first chord of the song. Typically this will be the “Key” of the song. Go through the basic chords first (C, G, D, E) the large number of songs will start with those chords. If you are unable to determine the first chord (perhaps it is a more exotic chord) then you can figure out the root note (or the lowest pitched string) of the chord.
In the case of Hotel California, it is an F#, Then you can play the major version, noting it does not sound like the recording, then the minor version of the chord. So in this case the first chord of Hotel California happens to be an F#m. I would suggest starting with relatively simple songs from The Eagles or similar as they tend to use standard chord progressions and simple chord forms. Several popular Eagles songs are G-C-D or similar.”
Listening Carefully a Recording of a Song
Another technique, presented by Ultimate Guitar is as follows: “Well, I first listen closely a few times to the CD and get the general feel for the song, the layout, the sections, the different instruments etc. Then I sit down with my guitar and pick out the bass line, or even just the bass notes to the chord changes. This may take a while if the bass is tough to hear.”
Using headphones may also be an interesting part of this process. Using headphones, it is easer to create an environment where each note can be heard distinctively.
“Sometimes headphones help. The thing about bass notes is that there are no majors, minors, sevenths, suspensions or anything like that to confuse you when you’re just getting started. I just start at the open E and continue up and down the string trying each note until one fits. I usually try the common keys (E, A, D, C & G) first. Then I restart the CD and narrow it down even more, until I have the first note, then the second, then the third. This process usually drives my wife crazy because she has now heard the first part of this song 14 times. So you might want to consider headphones.”
Listening Carefully to What You Play
Listening to you own sounds is also important. Jon Rhodes  says that “When practicing scales or guitar melodies, sing or hum the note you are playing. Start simply and build up. After some practice you will be able to hum every note that you play at the same time you play it. This exercise can seriously help bridge the gap between the mind and the guitar.”
-  http://www.ultimate-guitar.com/lessons/guitar_techniques/playing_by_ear.html
-  http://www.ehow.com/how_4929970_play-guitar-ear.html