It may be difficult for the average consumer to determine this. Here are some tips that will help. 

You might get an occasional "fresh air" smell when you open the cover, but the off-gas system will take care of most of the free ozone and you won't normally smell the ozone much. 

Bubbles coming out of the ozone jet are not necessarily a good indicator, either. Any ozone system moves air and pumps it out the jet. If the ozone unit is working, those little bubbles will contain ozone; if the unit has failed, the bubbles will be nothing but air. In an Arctic Ozone or Coyote system, you'll only have bubbles when a pump is running. With Peak Ozone, you might see very fine bubbles all the time. So if you don't see bubbles when you should, suspect the circulation pump or air intake rather than the ozonator. 

Some ozonators have an indicator light; if the light is working, so is the ozonator. However, not all ozone generators have such a light! 

The best indication that your ozone system is working is clear, sparkling water with a minimum of sanitizer. But even if you find yourself having to add more sanitizer, there are many other possible causes other than a lack of ozone. 

If you begin to suspect that your ozonator might not be working, take a water sample to your dealer for a thorough test. Have the other possible reasons checked and eliminated before discussing a check of your ozonator. 

UPDATE 06-Feb-08: A company called Pool Check offers Ozone test strips and a test instrument called Exact 9 that includes a test for ozone. These products are intended for swimming pools and we have not examined their suitability for use with our product. Arctic Spas does not use, sell, endorse or recommend these products, which are listed here for information only.