Cartridge stylus for sony record player


I am new to audio. I have a sony fully automatic turntable,
with a grado green or black cartridge which hasn't been used in a long time. How do I determine when to replace the stylus or cartridge?

How do I find replacement stylus?
samuellaudio
I would contact either EWsaunders.com or theneedledoctor.com
Both are great. By the way, if the needle sounds good, don't change anything.