NOVELTY - Method to determine the identity of a functional fluid comprises: adding a marker component to a functional fluid; using the marked functional fluid in an application; obtaining a sample of the fluid before, during or after the fluid's use in the application; placing the sample of fluid and a reagent solution on a test medium; reacting the marker in the functional fluid sample on the test medium with the reagent solution to produce a visible change; analyzing the results of the reaction by determining or comparing the resultant visible change; and determining.    USE - The method is useful for determining the identity of the functional fluid, where the functional fluid is automatic transmission fluids, engine oils, traction drive transmission fluids, manual transmission fluids, power steering fluids, antifreeze fluids, lubricating oils, greases, crankcase lubricants, mineral oils, oils with Group 1 ,2, 3 or 4 base oils, vegetable oils, differential lubricants, turbine lubricants, gear lubricants, gear box lubricants, axle lubricants, brake fluids, farm tractor fluids, transformer fluids, compressor fluids, cooling system fluids, metal working fluids, hydraulic fluids, industrial fluids, passenger car fuels, diesel engine fuels, bio-based fuels, continuously variable transmission fluid, and/or infinitely variable transmission fluids (claimed), the functional fluid is useful in a variety of automotive, off-highway vehicles, on-highway vehicles, equipment, machines, metal working and industrial application. The identity of the functional fluid is useful to prevent the improper utilization or unauthorized counterfeiting of the functional fluid.    ADVANTAGE - The method is safe, simple, rapid, accurate and easy to determine the identity of the functional fluid. The method does not require the use of absorbent material, which is used for checking the functional fluid, including wipes, shop towels, paper towels and napkins as the functional fluid can be used on any surface type material as long as the surface does not chemically interfere with the marker and reagent. The method determines the identity of the functional fluid, which helps to insure the good condition of the device and/or equipment containing the functional fluid.    DETAILED DESCRIPTION - Method to determine the identity of a functional fluid comprises: adding a marker component to a functional fluid; using the marked functional fluid in an application, where the marker component survives the service conditions of the fluid; obtaining a sample of the marked functional fluid before, during or after the fluid's use in the application; placing the sample of functional fluid and a reagent solution on a test medium such that they are in contact with each other; reacting the marker in the functional fluid sample on the test medium with the reagent solution to produce a visible change; analyzing the results of the reaction by determining or comparing the resultant visible change; and determining the identity of the functional fluid. An INDEPENDENT CLAIM is included for a diagnostic kit, for the analysis of marker, containing functional fluids comprising a medium, a liquid, a semi liquid, an aerosol or mist containing a reagent solution comprising a mineral acid, an organic acid, an organic base, a mineral base, an oxidizing agent, a reducing agent, and/or a chelating agent; and a method to determine the identity of the fluid comprising instruction, pictures, drawings and/or photographs.