You could try one of these

1. Estimate the amount of hydroxyl groups in the sample using chemical reagents (e.g., tricholorosilane, acetic anhydride, phthalic anhydride etc.)

2. Using infrared spectroscopy and Beer–Lambert–Bouguer law

Have you considered a titration method?

https://www.astm.org/Standards/E1899.htm

https://www.researchgate.net/post/Whats_an_accurate_method_of_determining_the_number_of_hydroxyl_groups_on_the_surface_of_a_porous_material