A simple and versatile strategy, denominated Slip Trace - Modified Lattice Rotation Analysis (ST-MLRA), is presented to enable the identification of the active slip systems in polycrystalline alloys from surface information. The slip plane trace orientation is used to define the potential active slip planes while the actual slip direction within the active slip plane is obtained from the grain rotation, as indicated by the stretching of the trace of the grain orientation in the pole figure as a result of deformation. Examples of application of the strategy in a HCP Mg alloy are presented for illustration. They show that the strategy is simple to implement and allows to identify the active slip system(s) in each grain.