Having played 34 years, I just have the sounds of the stings in my head. If someone has cranked it way out, I can quickly crank it into tune without a reference. I play church organ and keyboard periodically and therefore hear a correct A-440 often. A person playing instrument that is tuned below or above concert pitch with come to think A-440 is something different. Training that ear recognition isn't emphasized with guitar students, but a violin student is handed an A-440 turning fork immediately as a violinish has to play with the correct pitch in mind. Open strings are rarely used since one cannot use vibrato on them.

If I want to get finicky about a solo piece I tune octaves on G open 3rd string to the G's 3rd frets of the 1st and 6th strings. I then tune a perfect 5th (a nice clear hollow sound) between G open 3rd string and D at the 3rd fret of the B string. Open 5ths are actually clearer to hear than unisons or octaves because you listen to the strings beating together very easily. That D at 3rd fret on the 2nd string then serves as a reference to D open 4th string. D, 5th fret of 5th string is tuned in unison to D 4th open string. The perfect 4th relationship exists between G open 3rd and D open string below as a crosscheck.

I use D tuning fairly often (6th string lowered to D from E). Here I tune D octaves between 6th open, 4th open, and 2nd string 3rd fret D. I then tune a perfect fifth A open 5th string above the sixth. A perfect about D 4th string for A 2nd fret of the 3rd string. and finally A 5th fret of the 1st string is tuned a perfect 5th above the D 3rd fret of the 2nd string. A nice cross check is to sequentially pluck all strings while fretting the upper three strings to the A's and D.

Use of 5ths is worth learning if you play melodic work on the guitar and delicate chording. A fifth is the overall distance of the triad and essential a clear chord. The third may be brighter or darker, but the fifth sound need that perfect distance. Since I play in the key of G and C predominantly for clawhammer picking, I choose to work with the G. If I play in Keys of A or E then I will work with the 5th relations of A and E. The D tuning about, likewise focused on 5th relations of D.

Another worthy point to consider is that the primary 3 chords in western music are the major chords built on the 1st, 4th, and 5th notes of the scale, (Key is C: C-F-G) (Key is E: E-A-B) (etc) Check it out, all those prime tones lie a fifth away from the Key chord, (or a fourth, the inverse of a fifth, away from the Key chord) A tuning which perfects the important fifth/fourth relationships of the key, results in not simply an accurate tuning but and enhanced tuning brightness. Minor keys follow the same prime relationships. Modal keys occassionally are supported by fifth tuning.

Beginning or early guitar plays should always be issued and electronic digital tuner to begin training the ear immediately. Tuning a guitar is nearly impossible for a beginner and can kill the interest in playing. The tuners are a gift from the Muses.

Some players sound the prime chords to double check a tuning, This is a good technique, but what they are looking for is the perfect fifth, whether or not they realize it. The chord however is also sounding the 3rd of the chord which clouds the beating of the fifth.

Beating; In case some of you aren't familiar with the term, it is coincidence of vibrations between different notes. Octaves are a 2:1 ration (A-220 vs A-440 vs A-880) (C-523 vs C-1046)(ect) Hear the one vibration of the lower note coincides with two vibrations of the upper note, a 'perfect' relationship. The fifth relationship is 3:1. A unison is 1:1, obviously since it is the pitch to itself. On good instruments or single strings of a piano, your ear can pinpoint these perfect distances because it can hear these coincidental beatings. Anything slightly off in either direction is immediately very harsh. The ear in this capacity is far more accurate than a tuning device which samples sound and compares it to a programmed value.

Another popular tuning method is to play harmonics, usually using the octave harmonic at the 12th fret. Just touch the string lightly to deaded the fundamental string sound and you'll hear the harmonic. The 12th fret cuts the string in half, the 2:1 relationship of the octave. Touching the string lightly at the 7th fret divides the sring at the 1/3 lenght point and sounds the fifth harmonic. You can also find this same harmonic down towards the soundhole when you touch at the 1/3 lenght point from that end, which is interesting but inconvenient. Depending on how refined your guitar is, at the fifth fret you can find the 2 octave harmonic since you are touching at the 1/4 length of the string, a 4:1 relationship, which = two 2:1 relationships. The overtone or harmonic relationships then go the to major 3rd, the minor 3rd, ect indefinitely, but you can't produce on a guitar. (Get a trumpet or any brass instrument, they play nothing but harmonices.) Some guitar players like to test with harmonics because that measures the pitch of the string without problems of pressings, loose windings, warped necks, etc.
I never found it that useful, but it's fun to listen to harmonics.

Last point: since I rarely change my strings they are all worn at the fret contact points and being not uniform, really won't tune properly at all. Anyone who wants to sound their best at a performance or recording session, should change the set of strings first. Worn strings can be turned as open strings, but every playing at every fret will be off according to the damage of the string. Of course, a broken string is not a sound problem at all, so those don't need to be changed.