The hookup is only half the install. Once water runs through a new softener, the unit still needs its own settings matched to your water and your household, and someone should watch it complete its first regeneration. A unit can be plumbed correctly and still deliver hard water at the tap if the settings are wrong.
Before the installer leaves, get these numbers in writing
Ask for the hardness value the unit was programmed to, the unit of measure, and the water test it came from. A number copied from an old report or a guess is not the same as a reading taken on the water entering your home. Ask which regeneration type the unit uses (time-based, or demand-based using a meter) and the time of day you want it to run.
Ask for the salt type recommended for this unit and the fill level at handoff. Note where the brine tank float sits when the tank is full, so you can tell later whether the level has dropped.
Confirm the drain and the overflow before the first cycle
Regeneration pushes spent water to the drain. Check that the drain line is not kinked, that it has a proper air gap, and that it empties to a floor drain, standpipe, or other approved point rather than a spot that can back up. Ask where the overflow goes if the brine valve sticks open.
Run the first regeneration and check it
Ask the installer to start a manual regeneration while they are still on site so you can see the full cycle. Note how long it runs and whether the drain flow is steady. Afterward, test hardness at the cold tap nearest the unit with a test kit. If the reading is far from the programmed value, do not assume the unit is fine. Call the installer back before you rely on it.
Log the start-up values
Keep a card or note with the programmed hardness, regeneration type and time, salt type, the day of the first test, and the result. Those numbers are the first thing a service technician will ask for, and they make any later problem faster to diagnose.