The EEC can tell you flat out they are bad via the Malfunction Indicator Light (MIL), had that happen once.
It also shows up as poor fuel economy, loss of power, and poor throttle responce without a MIL code.
Edit: I would consider them a wear item to be replace every 60K or 6 years.
[ November 19, 2002, 05:39 PM: Message edited by: Robert Duncan ]