Before the computer gets a chance to take over again, snap the throttle
wide open while its still full lean and release to idle position, and see
how fast it rises to its full rich reading. If it gets there in less than 100ms,
the sensor is adequate. If you want precise control look for faster than that.
The image below illustrates how the signal will look. If it can't get to less
than 150mv, or more than 800mv, or if the lean too rich speed is more than
100ms, the sensor is due for replacement.

I suppose there's a fine line here, if the numbers are close, and its switching
good, I suppose you could still call it adequate and fuel control would also
be adequate. But if you are dealing with emissions testing, you will need to
get the fuel control as tight as possible. If the sensor can't tell the computer
the exact quantity of what is flowing past it, or takes forever doing it, the
computer can't control fuel fast enough to permit the Catalytic converter
to reduce emissions efficiently. (can cause efficiency codes)

Go to this page to see the test in action.

This test can be performed on the sensor installed at the outlet of the
Catalytic converter to verify its condition as well. These are looking at
Oxygen levels on the outlet side of the Cat, and should remain flat-lined
if the Cat is working efficiently. You could look at both sensors
simultaneously while doing the propane test, get them both flat lined
rich and record the max reading, then turn off the propane and see if
they both respond, if they do, go ahead with the wide open stab of the
throttle and look at the waveform, The front one should appear as
above, the rear one should appear as below, you can see the min,
max, and rise time.

When you start verifying Oxygen sensors, you will save a lot of time
not replacing them in hopes of correcting some drive-ability problem
you can't seem to nail down. Once you know the sensor is good, you
can forget about it being the cause of the problem you are trying to correct.