How can I measure amount of ozone byproduct coming out of hydroxyl generator
Ozone is very reactive, so unless you have an industrial scale hydroxyl generator, the ozone will readily react with organic matter in the air and surrounding surfaces before it builds up to dangerous levels. There are commercial portable instruments that are sold to monitor ozone concentration, they will be in the \$500 to $1500 price range for the instrument. You can also look at this simple experimental apparatus described in the Journal of Chemical Education for a cheaper alternative.