A colorimetric test for determining the presence or absence of biodiesel in a diesel fuel sample. The colorimetric test involves adding concentrated sulfuric acid to a sample of diesel fuel and observing any color change of the sample. A darkening of the diesel fuel sample indicates the presence of biodiesel. The degree to which the color changes after the addition of the sulfuric acid is proportional to the amount of biodiesel in the sample and therefore can be used to determine the amount of biodiesel in the sample. The reagent(s), sample containers and any necessary color scale can be provided in a kit for field testing.