TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The sample of the marked functional fluid and the reagent solution are placed on the test medium in any order, where the reagent solution comprises a mineral acid, an organic acid, an organic base, a mineral base, an oxidizing agent, a reducing agent, a chelating agent, and/or a metal salt, and the reagent solution and sample of functional fluid are placed onto the test medium using a dropper, pipette, squeeze container, pour container, pump without propellant, spray, roller, brushes, and/or dipping container. The step of determining the identity of the functional fluid comprises visually comparing the sample against a set of comparative visual indicia depicting the functional fluid and at least one different fluid as a guide, using printed instructions as a guide, and/or comparing photographs depicting at least one different fluid. The functional fluid is exposed to heat, and/or physical stress, during its use in the application. The application of the functional fluid is in an internal combustion engine, where the functional fluid is engine oil. Preferred Components: The marker component is added to the functional fluid before or during the fluid's use in the application and the marker is selected to survive the fluid's use in the application. The marker compound is added to the functional fluid in the form of a concentrate comprising the marker compound and a solvent, where the marker compound is miscible in the solvent and the solvent is miscible in the functional fluid, and the marker compound is substantially immiscible in water. The marker compound has a structure of formula (I). The marker compound comprises 4-dimethylaminoazobenzene, 4-diethylaminoazobenzene, p-dimethylaminoazobenzene-o-carboxylic acid, 2-aminoazotoluene, thymol blue, thymolphthalein, and/or malachite green carbinol base, and the reagent is a solution comprising a reagent solution comprising hydrochloric acid, a reagent solution comprising potassium hydroxide, a reagent solution comprising a mixture of hydrochloric acid, isopropanol and 2-ethylhexanoic acid, and the reagent is present at 0.001-5 wt.% of the reagent solution, and the marker compound is present at 10-1000 ppm of the functional fluid. The medium is compatible with the reagent indicator and the medium comprises paper, cellulosic material, chromatography paper, filter paper, polymeric fibers, natural fibers, fabrics, polypropylene woven fabric, non-woven fabric, metal, glass, plastic, and/or composite materials, where the analysis occurs after an effective period to allow for the reaction between the marker in the functional fluid and the reagent solution. The test medium comprises a substrate containing the reagent solution, where the reaction between the marked functional fluid and the reagent takes place when the sample of the marked functional fluid is placed on the test medium. The indicator in the reagent solution produces a visible change in response to reacting with the marker in the functional fluid. The kit further comprises a set of comparative visual indicia comprising two or more representations of the reaction of a functional fluid with the reagent disposed upon a same, similar or different media, where at least one representation shows the visual response expected from the marked functional fluid after reaction with the reagent and at least one of the other representations shows a visual response expected of a different functional fluid, after reaction with the reagent.R1 = H, -COOH, halide, nitro group, cyano group, azo group, sulfonic acid group, phosphoric acid group, amino group, alkoxy group, ester group, amide group, trifluoromethyl group, phenyl group, or alkyl group; andR2 = H, alkyl group or any other hydrocarbyl group, where any alkyl groups present independently contain 1-10 carbon atoms.