In the diagnostic cluster there is one for the oxygen sensor. You can put a multimeter across it to check that the voltage is fluctuating at idle. The car has to be warmed up a bit. Also when you rev it the voltage will swing to one side.
Check the diagnostic codes also to see if there is a problem with the sensor.
The car will still run with a malfuntioning O2 sensor, though mixtures would most likely be rich and you won't be getting the best performance.