Hard water is water carrying a lot of dissolved calcium and magnesium. It is not something you can see in a glass, but it leaves evidence wherever water dries or gets heated. The signs below are the ones a homeowner can notice without tools. Some of them overlap with other water problems, so the later sections show how to sort them out.
Signs on fixtures and dishes
Look for a white or gray crust on faucet aerators, the small screen at the tip of a faucet, and on showerheads. Shower glass and tile that stay cloudy after cleaning, and a film that builds along the waterline of a tub, are common. Glasses and dishes that come out of the dishwasher spotted or filmy, even when the detergent and rinse aid are fine, point the same way.
Signs you feel when you use the water
Soap that will not build a lather, or that leaves a sticky scum instead of rinsing clean, is one of the most typical signs. Many homeowners find they need more soap or shampoo than they used to. Laundry can come out stiff, and towels can feel rough over time. Water that dries on the hands without rinsing clean can leave a residue behind.
Signs inside the appliances
Scale collects wherever water is heated. Check a kettle, the heating plate of a coffee maker, the heating element of a dishwasher or washing machine, and the water heater. A water heater that makes popping or rumbling sounds may have sediment or scale in the tank, and that is a question for the technician to answer, not something you can confirm yourself. A showerhead that slows down over time, while the pressure at the main has not changed, often has mineral buildup inside its spray holes.
A simple check you can do yourself
Unscrew a crusted aerator from the faucet and soak it in plain white vinegar for an hour or so. If the white deposit softens, dissolves, or fizzes, mineral scale is the likely cause. Then compare what you see with the other signs. White or gray crust together with soap problems leans toward hard water. A rust-colored or yellow stain points toward iron, which needs a different look. A metallic taste or stains on white sinks and laundry that do not respond to vinegar also call for a different check. A rotten-egg smell is a separate sign with its own source, and it should not be lumped in with mineral buildup.
What to ask before you choose a fix
Hard water is described by a hardness figure, and that figure is what usually sizes a softener. Ask the company whether they tested the cold tap at your house or a sample taken somewhere else, and whether the number appears in writing on the report they give you. Ask how they will confirm the figure after installation, and whether the reading will be taken at the cold tap, since hot water can behave differently once it has passed through the heater. If a company recommends equipment without a measured hardness figure, ask what the recommendation was based on.
Keep a short note of which fixtures are crusting and when the soap problems started. Those notes help you describe the problem clearly to a company, and they give a technician something to compare against at later visits. The award methodology page explains how the companies on this site are evaluated.