I'll also try an unpack some of this:
To dumb it down a bit, ozone lines tend to have bubble coming out of them. Ozonators tend to have a life spa- 12-18 months for UV, 18-24 months for CD. There are variations of these, and these variations can increase in the ability to sanitize the water, or increase the lifespan of the generator. When the unit stops producing ozone, the only way to notice is to check the actual ozonator to see if the bulb is burnt out, or check the chip, etc. In most cases, the bubbles will continue, regardless of whether the unit is working.
If you're really into ozone, you can notice the smell of the ozone when you open the cover. That is always how I know whether or not it is working.
The quality of ozone has many layers: How slow is the GPM of the ozone line tends to be the best indicator. If the water is traveling too fast, ozone is not going to be effective. Also- The length of plumbing will be an indicator of how well the ozone performs. Also- The increased size of plumbing, or extras like mixing chambers, etc. can increase the effectiveness of the ozone.
Circulation pumps as a whole tend to be the better route, as others have explained. It is a slower motor that runs 24/7. But there are no ozonators that render your tub "Chemical-free," just as there is no way to treat a hot tub that is "Chemical-free."
Think of an ozonator just in the same fashion as your filtration system: They can vary in quality, and can vary in their ability to help you keep your water clean, but they are only a small part of that, and in no way are a substitute for water chemistry, sanitation, etc.
I hope this helps. Good luck moving forward.