If you disconnect the signal lead with the car fully warmed up it should read greater than .45V. The best test is to hook up a scope in parallel with the connected sensor so you can inspect the actual waveform. It should cycle between about .15V and .85V about one to two times per second at idle with a nice crisp jump between the two voltage levels.

A proper duty cycle also indicates a good sensor, but an anamolous duty cycle could indicate several problems.

When I do diagnostics, I usually test the O2 sensor first, and then look at the duty cycle.

Duke