Optically determining whether metallic features in different layers in a structure are in electrical contact with each other. When the metallic features include different metals and/or have different dimensions, which cause one or more resonances in reflected radiation to be detected, the metallic features in the different layers are determined to be in contact or out of contact with each other based on the spectral positions of the one or more resonances. When the metallic features are formed from the same metal and have the same dimensions, the metallic features in the different layers are determined to be in contact with each other responsive to detection of a single resonance associated with the metallic features and out of contact with each other responsive to detection of two or more resonances associated with the metallic features.