No real good test that I know of. One could watch the voltage, but the output just moves a few tenths of a volt and should cycle up and down based on fuel input. (i.e. closed loop control) The o2 sensors are ignored on cold start up for some period. ( seconds, minutes ?) , they are the 4-wire heated type so thecomputer can count on reading them sooner. Does it not miss during the warm-up period ?
I've had a few Fords over the years, and consider the o2 sensors as tune up Items at 80k or so, seen the effects of "Lazy" response that stays within
Computers OK range, put messes with drivability/response . The Ideal situation is if the light comes on,as the system has 4 o2 sensors. Only the front (upstream) sensors are for loop feedback, the other 2 ensure you cats are working. (OBDii self test). I read of the perils of people throughing hundreds on not need sensors. But the o2's do wear out,some sooner than others. On early Expeditions, the passenger side ones fail due to A/C water dripping right on top of them. On a 5.4 it is a near impossible tight job.