Should you have access to Photoluminescence, then one can look a the PL spectra to find new defect induced transitions, and correlate PL intensity with induced defects.  

Thanks Prof Roughani for your nice discussion kindly tell any more methods also. 

Photoconductivity will reduce.

Thanks Mehra sir for your nice answer. 

Raman measurements would be helpful.

Many good answers are already provided. I belive the UV spectrum is good, Further, you check the R.I. and ESCA, Laser Raman etc. If it is proper wavelength for grating formation you can check also diffraction efficiency.

Photo conductivity, Photo luminescence are useful. Please see the attached reprint of the work by Lucas et al. 2003, JNCS.

Thanks Prof Rashmi, Prof Samui and Prof Parthasarathy for nice discussions. 

Another direct method is using DLTS if you have access to this technique.

Thanks Mehra sir 

Welcome.

You can estimate the light induced defect in terms of " thermally stimulated current (TSC) measurements".

Please, open the attached file

Thanks Prof Shaaban for your nice discussion and attachments.