How do you know when to replace the stylist?

Usually it starts out with a slight degradation in cymbals, or some sybilance in sounds, then outright distortion with various types of music. It will appear at first as if the needle is dirty and trying to clean it will not change the sound. Try to imagine your needle full of sludge and worn down trying to play a groove,,,that will sound bad.
Ciao,
Audioquest4life

I second what audioquest 4life says. I am in the process of replacing my current cartridge. I started loosing detail in the high frequencies and certain passages started sounding more sibilant, and some music would just totally distort to where I couldn't listen. Basically the cartridge couldn't track well any more. It sounded like bad inner groove distortion.

At one time: Every serious audio salon(not going back to the "stylist" thing- honest) had a microscope which which to inspect your stylus. I'm certain(if you live near a metropolitan area) you could still find one. Really- any decent microscope would do. Just look for wear at the points of contact with the groove walls. Here are a couple stylus retipper tips:(http://www.vandenhul.com/Default.aspx?page=p&id=29) (http://www.sound-smith.com/retip/) If you have a cartridge that's honestly worth the effort Van den Hul is the bomb!! Plug, "stylus retipping" in your searchbar for more options.

OH- On the Van den Hul site: You have to click on "phono cartridges"(left of page), then "extra services" at the bottom of the cartridge list.