How do you tell your Cartridge needs replacing?

I know this is a stupid question but I don't know how to answer it. I have a Sumiko Blue Point No. 2 on a MT-2 player. Came with it. Probably , 4 years old now. The first 2 years I was not cleaning records but have been for the past 2 years. I have no equipment beyond my ears to measure degradation of the stylus. Seems to me that the intervals between cleaning of the stylus due to muffled sound are getting shorter. That is all I can say. Maybe my brain is adapting to the sound degradation over time and what I would not consider abnormal 4 years ago is now normal. Anyways, I suspect the easy answer is just to replace it and listen but was wondering if there is any other advice out there. Thx.