The install is not finished when the old unit is gone and the new one is bolted down. It is finished when someone has shown you how the system runs, where you can shut it off, and what is written down for the next person who has to fix it. Most of the questions you will have later get answered in the last stretch on site, or they don't get answered at all.
Ask for the bypass first
Before anything else, ask the installer to show you the bypass valve and the shutoffs on the supply lines. Have them put the softener into bypass, let you see where the water goes, and then return it to service. You need to know which handle does what, because the first time you call about a leak or a plumbing job, you may be standing in the utility area with nobody else around. Confirm that you can tell the inlet and outlet valves apart on both sides of the unit.
Walk the control display with them
Ask them to read every setting on the control head out loud, then compare each one to the install sheet. The items to check are the hardness setting, the regeneration type, the time of day the regeneration is scheduled to run, the salt dose setting, and the capacity figure the controller is using. If a value on the screen does not match the paper, do not accept 'it will adjust itself.' Ask them to correct it while you watch, and ask them why the number was set where it is.
Confirm the drain and the salt
Have them point to the drain line and show you where it ends. Ask whether the drain connection matches what your installation paperwork describes, and whether the line is secured so it cannot slip out during a cycle. Then ask where the salt goes, how full the brine tank should be, and how you will know it is running low. If the unit has a brine level indicator, ask them to show you what it reads when the tank is full and when it is low.
Take the papers that belong to you
Before the truck leaves, collect the owner's manual for the control, the warranty certificate or card, the final install sheet with the settings written in, and the name and phone number of whoever you call when something goes wrong. If anything is missing, write down what is missing and get a delivery commitment in writing for when it will arrive. Keep these together in one folder. That folder is what the next technician will ask for first.
Run one cycle and watch it
If the installer can start a regeneration while they are still on site, ask them to do it and stay for the first part. You do not need to understand the chemistry. You need to see the unit move through its steps, hear whether the drain is flowing, and confirm that nothing is spraying or pooling. If the crew cannot run a cycle before leaving, ask them to tell you exactly how to start one yourself and how to tell whether the automatic one has run.
Sign off only on what you saw
A hand-off is a short checklist, and you should be able to tick each line with something you observed rather than something you were told. Ask the crew to note on the install sheet that the bypass was shown, the settings were read back, the drain was checked, and the papers were handed over. If something was skipped, have them write that down too. A note that says what was skipped is more useful to you than a clean sheet that hides it.
If you are still comparing installers and want to know what we look for at this stage, the checks are explained in How We Rate Water Softener Companies.