An apparatus for and method of determining the states on a wafer to be processed, e.g., whether residue in the form of metal is left on the surface of a wafer after chemical-mechanical polishing. The method comprises the steps of calculating first spectral signatures from a first set of measurement sites on one or more training wafers. Each measurement site is known to be in one of two or more states. In the case of only two states, the states could be "residue present" and "residue absent" states. The next step involves correlating the first spectral signatures to the states on the training wafer(s). The next step then involves calculating second spectral signatures from a second set of measurement sites on a wafer where the states are unknown. The final step is determining the states on the wafer to be processed based on the second spectral signatures.