Most homeowners learn their water is hard by accident, when glassware comes out of the dishwasher cloudy or a shower door starts to look filmed. Before you pay for a softener or sit through a sales visit, you can get a rough answer yourself in about ten minutes. It will not give you a number to size equipment with, but it will tell you whether the question deserves a proper test.
The lather test
Use a clean clear bottle or jar. Fill it about a third full with cold water from a kitchen tap, add a few drops of the liquid soap you normally use for hands, close the lid, and shake for about ten seconds. Then look at the surface. Soft water tends to produce lather that holds for a while. Hard water tends to produce less lather, and you may see a thin film or small floating curds. Repeat with a second sample from a different tap, such as a bathroom sink or an outdoor spigot if it is connected to the same supply, to see whether the result is consistent through the house.
What the test can and cannot tell you
This is a comparison, not a measurement. Soaps differ, and some detergents are made to lather in hard water, so a good result does not prove the water is soft. Hardness is reported in grains per gallon or milligrams per liter, and only a lab or a calibrated test kit gives you that figure. If the lather test is unclear, or if you plan to spend real money, get a certified water test. That reading is what a company should use to size the equipment.
Other signs that point the same direction
Mineral film on glassware and fixtures, stiff laundry, a scum ring in the tub, and using more detergent than you expect all tend to show up together when water is hard. Each one also has other causes, so count how many appear at once instead of treating any single sign as proof.
Rule out iron before you blame hardness
Reddish or brown staining, a metallic taste, or orange residue in the toilet tank points to a different problem. A standard softener does not remove iron and can be fouled by it. If you see these signs, tell every company before they quote you, and ask them in writing how they would handle iron.
What to bring to the next conversation
Write down your lather results for two or more taps, any hardness reading with its unit, whether you see rust-colored staining, and anything you have already tried. Give the same notes to each company you talk to. A company that recommends equipment without asking for these details has skipped the step that matters most.