O2 ?'s

[DandD]

1988 300-SEL
O2 senor -
What are indicators of O2 problems?
Do they need to be changed on regular basis - milege?
If so, are there other things that need changing at same time?
What?
Are there test that can be done to detect if O2 is bad or going bad?
What are the steps to changing one?

Thanks!