Speaking of natural sound. I consider un-amplified sound from various musical instruments as my reference.
As we have an upright piano, an organ and a guitar I always listen to the notes that these produce. When there is an opportunity to listen to bands during fiesta, church celebration, school celebrations etc I make it a point to listen properly. Through time I have "memorized" the various notes of the musical instruments.
I compare the notes I have "memorized" vs the notes I listen during sessions. Depending on how far the reproduced sound is that I make my judgement.