Connect an XLR to 1/4-inch adapter to the end of a microphone cable. This will allow you to use a microphone with a standard tuner. Although you would usually just plug the guitar in directly, you can use this technique with an acoustic guitar or on an amp while recording in the studio.
Place the microphone in front of the speaker cabinet. Place it just off-center of one of the speakers a couple of inches away to get the best sound from the notes you want to check. Pluck a string on the guitar to see if the note is being registered on the tuner.
Pluck the lowest string and tune to pitch. You can pluck each individual string and tune each to pitch or simply tune the lowest string and tune the rest to that string. Make sure there are no other amps or instruments in the room with your microphone as it will pick up the other sounds and give you false readings on the tuner.
Connect the tuner to the microphone by using an XLR to 1/4-adapter. Plug the 1/4-inch plug directly into the tuner. This will allow you to tune an acoustic guitar using the microphone.
Set the microphone on a stand and bring it very close to the sound hole on the acoustic. Place it no more than 6 inches away, but not too close as to restrict the strumming and string plucking.
Tune each string to pitch individually, noting how sharp and flat each is on the tuner. Line up the needle to make sure that each note is in perfect tune. Play a chord or two in-between to make sure that the guitar sounds as though it is in tune as well. Although a tuner is able to help you tune each note, the sound of the open chords will let you know if you are actually in tune or not.