How to know when an audio component has degraded?

Aside from a catastrophic failure or an obvious issue such as a buzz, hum or static problem, how do we know when a component in the audio chain degrades?  My question centers on a simple yet delicate component - the phono cartridge.  I'm currently using a 4 year old Dynavector DV20x2.  It has been a great cartridge but I wonder if it has degraded in any way.  My perception is a slight loss of high frequency resolution but I have been listening to a lot of digital lately and wonder if the cartridge output has changed or if my expectation has changed in some way.  The question could apply to any component in the audio chain.  Lets face it, audio components are complex and our ability to differentiate subtle changes over time diminishes as well.  A slight drift in the frequency response of a component or a more complex problem such as a phase shift that varies with output level is very difficult to quantify.  We want to think our equipment remains stable for a long time after we make the purchase but in reality, everything in the system is in some state of change.  I am posting this as an analog question since I am looking for a way to know if the performance of my phono cartridge has deteriorated.  Some will suggest tracking the hours of use but that says very little about how it sounds.  Some may even suggest having a reference component to compare.  That method may be too expensive for most and certainly inconvenient for all.  As far as the cartridge goes there are no suitable, by that I mean practical ways of maintaining a reference.  The same is true for every component in our systems.  It may in part explain why we tend to change components out so often.  Is it that "we" simply get tired of the sound we have and at times a different component sounds better or is it that our components got tired and "we" as the ultimate reference noted the change.  I guarantee a truly great system will sound great for a while.