A method for automatic determination of a state of a substrate in a plasma processing chamber is provided. Substrate reflectance data is collected in a processing chamber prior to processing to be analyzed with reference reflectance data to determine if the substrate state meets a control criterion. The substrate state may define the thickness and the qualities of the films on the substrate, the critical dimensions of the different layers on the substrate. The reflectance data is analyzed using a multi-variant analysis technique, such as principle component analysis. In addition to analyzing substrate state prior to processing, substrate reflectance could also be collected in a processing chamber during processing to be analyzed with reference reflectance data to further determine if the substrate state and/or the substrate processing are meeting a control criterion.