To your second point, you can't with any certainty. I suppose your initial comparison many hours previously could be to some control such as another set of headphones but even this is dubious IMO. The brain effect would trump any subtle change.
[size=x-small]To your first point, I can see where mechanical devices may change but I am a bit doubtful that [/size][size=small]electronics ([/size]discrete or ICs) change much over time. If significant changes can occur in electronics over time then you would have issues with your systems. Which occurred when tubes were an integral component in many systems. Tubes would change over time and move outside their specifications which you either designed for or your system wouldn't work correctly. During my stint through aerospace, I never heard anyone overly concerned about transistors and caps. They either worked or they didn't.