There are many emperical rock mass classification systems. For basic rock slope stability analysis of rock cut slopes along a hill area road, Slope Mass Rating (SMR), Geological Strength Index (GSI) etc. can be used. Before using or deciding the classification system to be used, it is expected to take a few field traverses.
"Engineering Rock Mass Classification: Tunnelling, Foundations and Landslides" (By Goel, R.K. and Singh, B.) [BH/ Elsevier] can be a helpful text book.
In my opinion, If you do not have the opportunity to drill and take rock sample in the your work field, I think GSI will be the most appropriate classification system for you.