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Well the 2 ways I know of on toyotas:
With the car warmed up, rev the engine and measure the voltage between the O2 sensor output and ground. It should oscillate several times a second as the engine overshoots and undershoots the "correct" fuel-air ratio.
The other way is to take it out, and measure the voltage on the output wire while heating just the sensing end of the sensor with a propane torch. You should see a voltage output, because, as far as I understand it, the O2 sensor is a battery which is powered by O2 concentration *differences* between the sensing end, and the "outside the exhaustpipe" end. Which is why those little holes are drilled in the side of the outside end: gives access to the atmospheric O2 concentration.
There is a O2 sensor problem that I don't think either of these tests will find, and that is when the O2 sensor develops a voltage offset which skews all the readings high or low.
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