IR reflection spectroscopy is a usefull technique for the detection of OH groups.

Raman spectroscopy also but is less efficient than IR.

To determine the OH groups on Silanol, FTIR method is ideal choice.

Depending on your instrumentation possibilities and the state of your sample, NMR can be a very accurate tool to perform a qualitative measurement. 

Regards. 

Thanks a lot. Still need more useful suggestions.

Using the TGA data you can calculate the Silnol group rougfly.

Thank you all. Thank you Professor Lambert with your useful comments.

If you want to compare the silanol group fo each sample relatively you can use TGA method because it is simple  and easy way for experiment.

Calculation of the ratio of the integrated intensity of peak at 3750 cm-1 to integrated intensity of peak at 1865 cm-1 (corresponded to the Si-O of bulk silica) from IR-spectra and determination of the specific surface area by BET allow to estimate the loading of silanol (OH) groups on the surface of the silica

Silanol (OH) groups on the surface of the silica gel - quantitatively with MeMgJ. You measure volume of methane.