During polishing of a substrate a sequence of measured values is received from an in-situ motor torque monitoring system. Positions on the substrate of the region of lower coefficient of friction are calculated for at least two measured values from the sequence of measured values. A first measured value from the sequence of measured values at which the region of different coefficient of friction is at a first position in a first zone on the substrate is compared to a second measured value from the sequence of measured values at which the region of different coefficient of friction is at a second position in a different second zone on the substrate or is not below the substrate. Based on the comparison, which of the first zone or the second zone the overlying layer is clearing first to expose the underlying layer can be determined.