A-440 Tuning Fork
An A-440 Tuning Fork is the standard pitch used in most musical applications.
I have used this 'tuning standard' to obtain the reference note for tuning my pedal steel by ear for many years.
This 'Reference Note' is obtained by holding the tuning fork over the guitar pickup with the amp on, & then while engaging the foot pedal which raises the 3rd G# string to A on most E9th tunings, pick this string to make it sound & match it (removing the beats [waves] to the 'A' note of the tuning fork.
Keeping the A&B pedals engaged (making for an A Major chord in the open position on the E9th tuning) while matching the 'A' notes as per above, therefore accommodating for the possible 'body collapse' of your guitar, tune your 4th E string to the 'A' note removing the beats [waves] to result in a compensated (sweetened) sound which is pleasant to the ears.
An explanation of Al Brisco's 'Ear' Tuning Method is available on this SGC web site under Resources/Repairs & Tips.