I googled it and liked this answer:
In low concentrations it will smell like the the air does after a lightening strike. Kind of a fresh, watermelon smell. Put your nose over the water over the rising bubbles from the return. Is the smell more intense now? You could also try taking a plastic cup, put it under the water and allow it to fill with spa water; invert the cup under water over the discharge of the bubbles and allow the bubbles to displace the water and fill the upside down cup. Carefully lift the cup out of the water and quickly sniff the contents as it breaks the surface. The smell should be quite obvious if the generator is working.
My search was; "how do i tell if my ozone generator is working" and there were other answers.
As for me I researched it before and came away with the impression they generally last 2 or three years if you change the check valve annually, and use an ozone test strip kit to see if it has an output.
Easier now that I'm (inching) past the newbie stage, is hook it back up and chlorine demand should increase because ozone eats up some fc. I think mine eats up about 1ppm in 24 hours.