A useful tip when tuning the guitar is to listen for a vibrating or "wah wah" sound when two notes are ringing together. Each note vibrates at a different speed, or different amount of cycles per second. When two different notes are ringing together they vibrate at different speeds and cause a wah wah sound. This sound can be difficult to detect but if you play both notes loud enough, and you have complete quiet, you might be able to hear the sound, or even feel the sound as a pulse through the body of the guitar.
This vibration can be shown as a wave form. If the two notes played together are a long way out of tune with each other you will hear the wah wah sound vibrating quite fast.
If you do manage to hear the vibrations listen to the speed as you adjust the tuning key. If you hear the vibration increase then you are getting further out of tune, i.e., you are turning the tuning key the wrong way, If the vibration gets slower, meaning the wah wah sounds become less frequent then the two notes are getting closer together. The following waveform represents two notes almost in tune with each other.
When both notes are in tune with each other the wah wah sound should virtually disappear.