How can I tell if rough paint is caused by iron fallout, tar or other contamination?

Iron fallout often appears as tiny orange or dark specks and reacts with an iron remover, usually changing color as it dissolves. Tar tends to form black or brown sticky dots, usually low on the vehicle behind the wheels, while overspray can feel like a fine gritty mist across multiple panels on one side of the car. Visual inspection plus targeted chemistry is safer than immediately reaching for clay.