Knowing when to replace an oxygen sensor

[Wallace White]

Fellow listers -

Is there a way to really know when to replace your oxygen sensor? My
5kcstq uses the 3-wire variety, and I've tested what I know to test:
appropriate resistance across the two heater wires, and ~1 Hz voltage
swings about a 0.4V average while the engine is running.

Those tests seem to say that my oxygen sensor is fine, but I haven't
replaced it in the 30k miles I've had the car, and given what I know of
the PO's maintenance, it's probably very old. I know the right
answer--go ahead and replace it--but I like to know that the part I'm
replacing really needed it.

Thanks!

- Wallace
  '87 5kcstq 166k

P.S. Saw a Jensen Interceptor III the other day. Was it only the Jensen
FF that was all-wheel drive?

P.P.S. Saw an ~'82 Coupe the other day with a two-tone black and silver
paint job. Audi didn't deliver any that way, did they?