I've worked with ozone generator suppliers.
Usually, the ozone people measure the ozone indirectly, using ORP meters. ORP is oxidation-redox potential. The measurement is of the electrical activity of the ozonated water, in millivolts. A higher ORP reading = better sanitising effect.
ORP meters are expensive, but will give you accurate results.
You can also get paper test strips for ozone concentration, these are used for measuring water quality in swimming pools, spas (jacuzzis), etc. They aren't particularly accurate and might not work for the range you are targetting, but will be low cost.
I think if it was me, I would request to borrow your ozone generator supplier's ORP meter for the duration of your shelf life trials. They should also be able to tell you what ORP to use for your desired outcome.