Maybe The Beatles did sound out of tune, when hearing some of their records, but in these days of modern technology, when you can purchase any device to tune your instrument, using any mean at any price - The tones can sound virtually more accurate, than ever before!
Tuning a guitar with a piano
One way for tuning a guitar, can be using another instrument involved. A piano for example, can be a more accurate tool, when itself is tuned (if not, a piano tuner specialist can be called to do the job). Other Methods for tuning a guitar, in particular regarding the use of instruments, can be with a pitch pipe - this tool is a kind of harmonica, that with a single Exhalation, the player can tune each string, separately. For a guitar there is a six-tone model pitch pipe, used for a standard guitar tuning.
Using a tuner to tune a guitar
Another way for a simple tuning, comparatively, is using a snark tuner. A gadget that is clipped on the accoustic or classic guitar headstock (the guitar’s head) , and using a digital technology that relies on the strings vibrations, can show the player how to tune them. A device that is used by electric guitar players, is considered to be the tuner pedal - a tuner stomp box which is plugged to the guitar on one end, and to the amplifier on the other. This tuner pedal is used for jam sessions, and for gigs, and therefore it’s considered more proffesional!
Tuning a guitar by ear
Not the first method of having a tuned guitar for a beginner, but in time, practice makes perfect! For starters, a standard tuning is called E - for the low and the high strings, placing the B, the G, the D and the A strings between, counting them from the lowest to the highest (if you are right handed, that means). You can start by tuning the highest string, which is a low E (regarding the tone, not the position. Metallica’s “Nothing Else Matters” for example, begins with an open E...): put your finger on its fifth fret. If the lower open string sounds alike than they are harmonically adjust. You can do the same for each string below, on the same fret, except the G string, where you need to place you finger on its fourth fret, in order to adjust it to the string below (the open B string)!