This article and these links can be helpful for you.
Best.
Mithat
Beck, A. T., Baruch, E., Balter, J. M., Steer, R. A., & Warman, D. M. (2004). A new instrument for measuring insight: the Beck Cognitive Insight Scale. Schizophr Res, 68(2-3), 319-329. doi:10.1016/S0920-9964(03)00189-0
You can use parts of "Present State Examination" scale. It is most widely used scale. PSE is an interview schedule. It will also guide you how to ask questions to assess the level of insight.