Do it in the DARK! That’s right, you can use a ultraviolet light (black light) commonly found at your local hardware store to discover unforeseen oils and grease hiding on the surface. The hydrocarbons in grease and oil will literally light-up when you shine the black light on them. While this method works well on hydrocarbon oils, synthetic oils may not always show up.
Use this far-out psyhodelic clean screening method to also discover other items, such as fibers and particles that may have remained hidden to the naked eye.