If you have access to a scanner, like Jim mentioned the reading will fluctate rapidly on a good sensor. An error code will flash if the sensor stays high (.7 volt or more) or low (.2 volts or less) for more than 50 seconds.
If you have a digital volt/ohm meter you can measure this on the car. Another neat little tool I bought for mine is an adaptor from the aftermarket corvette suppliers. (like Midway)This plugs into the harness and allows the sensor to plug into it, that way you receive no error message.
Start the engine and let it reach operating temp. Shut it off. Disconnect the pigtail from the O2 sensor. Restart the engine.(you will flash an error, don't worry, you will clear this later.)Let it run for a minute or two and then take voltage measurements from the sensor. Take a continuous measurement for around 1-2 minutes and record the voltage output. Shut off the engine. Reconnect the O2 sensor. Disconnect the negative cable for about 30 seconds (to clear the error) and reconnect the cable.
If your voltage readings are sluggish, then it's a good sign that the sensor is starting to fail. If the reading fluctuates rapidly, then it's time to start looking elsewhere.
How many miles are on the vehicle? An O2 sensor should be replaced about every 45k miles.
67 Camaro LS6 454/TH400/12bolt 3.73
1989 TransAm 5.7L WS6 W/all the options