In Mix It Like A Record, Charles Dye recommends several methods of checking a mix for mono compatibility. The simplest: Put a finger in one of your ears!
There may be slightly more to it, however. Lifehacker recently featured a list of body hacks, and it included this great tip about using our ears:
If you’re stuck chatting up a mumbler at a cocktail party, lean in with your right ear. It’s better than your left at following the rapid rhythms of speech… If, on the other hand, you’re trying to identify that song playing softly in the elevator, turn your left ear toward the sound. The left ear is better at picking up music tones.
Neuroscientists have learned that because of the brain hemisphere to which each of our ears is connected, our left and right ears hear things differently. The right ear responds more to rhythm and speech, while the left ear is more attuned to music.
From a study published in the Canadian Journal of Experimental Psychology:
This finding provides support for claims of a right-hemisphere bias for the processing of melodic contour… In the present study, we sought to determine whether short tone sequences presented monaurally (to one ear at a time) are processed differentially by the two hemispheres. Studies of auditory processing often indicate that linguistic and musical stimuli are processed preferentially by the left and right hemispheres, respectively
This has an obvious implication for mix engineers: If you favour one of your ears, it could affect your mix decisions. Dye’s trick of checking a mix for mono compatibility could yield different results depending on which ear you stick your finger in!
Of course, you can use this to your advantage too. For example, when you need to make a decision about a mix’s rhythm elements, you might find it easier if you listen only with your right ear.