NOVELTY - Determining the identity of a functional fluid, comprises: (a) adding an optional optically active marker component to a functional fluid; (b) obtaining a sample of the fluid before, during or after its use in an application; (c) passing a beam of polarized light through the sample of fluid; (d) measuring the rotation of the plane of the light after it passes through the sample of fluid by comparing it to the light's original orientation; and (e) determining the identity of the fluid by the amount of rotation observed.    USE - The method and diagnostic kit are useful for the determination of the identity of a fluids, which are 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 1-4 base 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 (all claimed).    ADVANTAGE - The method: provides a convenient and reliable process for identifying the fluid before, during and/or after the fluid's use; and analyzes functional fluids rapidly in the field, and tests the identity of a functional fluid in the field rapidly by untrained personnel and without precision measurement. The diagnostic kit identifies the functional fluids rapidly in the field.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a diagnostic kit comprising a source of polarized light, a means for directing a beam of polarized light from the source through a sample of fluid, a means for detecting the amount of rotation in the plane of the beam of polarized light after it passes through the sample, and a means of communicating the results from the analyzer to a user where the kit also comprises written instruction, pictures, drawings and/or photographs to aid the user in the identification of the fluid.