Hard water leaves its marks on hard surfaces long before most people think to test it. The minerals that make water hard, mainly calcium and magnesium, come out of solution as water is heated or dries, and they form a chalky film or crust. Learning to read those marks lets you judge whether hardness is part of the problem and which parts of the house it affects.
Look at the faucets and showerheads
Check the faucet aerator, the small screen at the tip of the spout, and the face of the showerhead. A white or grey crust that builds in a ring, or that clogs the spray holes, points to mineral deposits. Soak one aerator in a mild vinegar solution, rinse it well, and watch whether the crust returns over the next few weeks. Wear gloves, and let a hot-water fixture cool before you handle it.
Check the kettle and the coffee maker
A pale scale line inside a kettle, or a crust on the heating plate of a coffee maker, is a common sign of hard water. Scale that rebuilds quickly after you descale it is a stronger sign than a faint line that never grows. Note how fast the buildup returns so you have something concrete to compare with later.
Look at the water heater and the drain valve
Sediment and scale can collect in a water heater over time, and many heaters have a drain valve that releases it. Only open that valve if you know how your heater is built and the power or gas is off as the manual directs. If you are unsure, have a plumber check the tank and tell you what they find at the bottom.
Separate hardness from other staining
White crust and scale point toward hardness. Rust-colored or orange-brown stains are a different problem, often linked to iron, and they do not respond the same way to the same fixes. Dark specks or slimy films may come from another source entirely. Do not decide the cause from appearance alone. A water test sorts these out, and the Missouri Water Softener Buyer's Guide explains the steps for reading results before you commit to a fix.