A softener exchanges hard minerals for sodium or potassium in a resin bed, then flushes the resin with brine to recharge it. The water looks the same at the tap either way. What changes first is what you see on fixtures, glassware, and the appliances that use hot water.
Early signs at the fixtures
Look for a film on glassware after a normal wash, spotting on shower glass that does not wipe away easily, or a slightly slick feel to water that used to feel clean. These point to changes in the water, not necessarily to a fault in the softener. Note them in writing as you see them, and compare them with how the taps and fixtures looked in an earlier season.
Brine tank behaviour
Check the brine tank level and the salt. A salt bridge, a hard crust at the surface, or a tank that stays full while the salt is not being used can stop the recharge from happening. Break up any crust with a tool made for the job, and keep the salt at the level your unit's instructions give.
Regeneration timing
Many softeners regenerate on a schedule or by water volume. Check the setting against how much water your household uses now. A house with more people, or a new water-using appliance, may need a different setting from the one the unit was installed with. Ask whether the controller clock was reset after a power outage, because a reset clock can run the cycle at the wrong hour.
Test before you adjust
Hardness can be measured with a home test kit. Test the water at the tap before and after the softener to see what the unit is actually doing. A reading that does not change across the unit points to a bypass, a resin problem, or a setting. Record each result alongside the notes you made on earlier checks.
Keep the routine short
Set a brief check you repeat on a regular schedule: salt level, crust, controller setting, and the feel of the water. The check takes only a few minutes and makes a change visible early. Age is one planning input for the unit. Your readings and what you see on the fixtures tell you when to act.