I am far from being an expert on neither -OH nor radicals, but is seems to me that since it is a radical, it is highly unstable and therefore any attempt to measure the concentration would yield a nonsense number. So I guess an (color changing?) indicator that would be highly preferred by the radicals generated in your system? It's just a thought though...

I support Kaja's answer. I would suggest to use EPR. One of the most often used spin traps for ROS detection is 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). DMPO is sensitive to OH• and  O2•− radicals.

Check my thesis and the article.

Look this article. 

Thanks all .. 

look this articlle

https://www.researchgate.net/post/Which_is_effective_method_to_determine_OH_radicals_in_photocatalytic_reaction#57b960263d7f4b92ee17e86b