Soaps are the sodium or potassium salt of a long chain of carboxylic acid. Detergents are ammonium or sulphonate salts of long chain hydrocarbons. Hard water contains chloride and sulphates of calcium and magnesium. When soap is added to the hard water forms less lather so some amount of salt is unused. This insoluble salt is known as scum. Detergent will give a good amount of lather in hard water as well as soft water. We can't identify through this whether water is hard or soft.