Many of us have this issue. Thanks for bringing it up. The easiest way is exactly the way you describe. Have a spare new replacement stylus, and when you feel that it's worn, swop and listen. If it sounds similar, swop back and store the new. If it sounds like it's time to change, then change and get a new one for reference as the control. I do exactly that.
Testing stylus wear.
I have an Ortofon 2M Blue stylus that I’ve had for 3.5 years. They advise up to 1000 hours of useful life but that’s just a rough estimate. I have not kept track of my hours of play. I’ll go for 3 weeks with lots of use and then a few more weeks of very little analog use and mostly digital use with my DAC. I just bought a lot of mint vinyl LP’s and got scared that my stylus may be worn, even though it sounds fine to me.
So, I have a new stylus and usb microscope on order. I just found an interesting article (http://www.pspatialaudio.com/stylus_wear.htm) that suggests something other than a visual inspection, since the tip is the only part of the stylus that gets worn and it’s apparently too small to judge even with a microscope.
The new method is to make two identical digital recordings--one with the older stylus and a second one with a new stylus. Then you A-B the two recordings. Since I already have a new 2M Blue coming tomorrow and another one on my turntable with unknown wear, I think I’ll give this method a try.
Has anyone heard of this? Any better ways to test if i should junk the old 2M Blue or somehow test it?