Well, sometimes it's very obvious, a bad smell coming from the water or pieces of sediment coming through the pipes. Other times it's more difficult to tell without testing because you have things like your hair/skin feeling very dried out, there's a tint on the inside of your sinks or tubs or even little white splotches on the faucets and handles. Now some of these things are minor and do not require water filtration, but most things like high iron in the water, hardness, or pH issues are and should be looked at to determine the severity of the problems.