The use of a transdermal Raman spectrum to measure glucose or other substance concentration can give an inaccurate result if the Raman signals originate at a wrong skin depth. To predict whether a spectrum of Raman signals received transdermally in a confocal detector apparatus and having at least one component expected to have an intensity representing the concentration of glucose or another skin component at a point of origin of said Raman signals below the surface of the skin will accurately represent said concentration, peaks in the spectrum at 883/4cm<-1> and 894cm<-1> are measured to determine whether the Raman signals originate primarily within the stratum corneum so that the spectrum will be less likely to represent said concentration accurately or originate primarily below the stratum corneum so that the spectrum will be more likely to represent said concentration accurately.