Local polydispersity refers to variety in the types of molecules present at the same retention volume in an analysis by size exclusion chromatography (SEC). Such variety is undesired because it can be a major source of inaccuracy in SEC interpretation. A rapid, practical, method for detecting the presence of local polydispersity is presented. In this method, data from a differential viscometer (DV) and light scattering (LS) detector are used together with a universal calibration curve to generate a differential refractive index (DRI) chromatogram for the sample, while assuming that it does not exhibit local polydispersity. This "reconstructed" DRI chromatogram is compared to the actual DRI chromatogram. It is shown that any significant difference between the two indicates the presence of local polydispersity. Plots of residuals (the difference between the heights of the two DRI chromatograms vs. retention volume) allow the significance of the local polydispersity to be assessed and the retention volume range encompassing the local polydispersity to be defined.