Can you use soaps and synthetic detergents to check the hardness of water?

(N/A) Soaps form a precipitate (scum) in hard water due to the presence of $Ca^{2+}$ and $Mg^{2+}$ ions,but they do not form a precipitate in soft water. Therefore,soaps can be used to check the hardness of water.
Synthetic detergents,on the other hand,do not form a precipitate in either hard water or soft water because their calcium and magnesium salts are soluble in water. Therefore,synthetic detergents cannot be used to check the hardness of water.