A new softener or filter often leaves the install crew with its controller set to a factory default, and nobody checks that default against your house until the first problem shows up. Before you sign the completion form, have the installer walk you through each setting while the unit is still in front of you. This is the cheapest check in the whole job.
Get the hardness number in writing
A softener's regeneration setting depends on how hard your water is at your connection. Ask which test the installer used, what the reading was, and what number they programmed. If the programmed number differs from the reading, ask why. Write the reading, the setting, and the test method on the card you will keep with the unit.
Check the regeneration mode and start hour
Most controllers can regenerate on a fixed clock schedule or only when the resin has used up its capacity. Ask which mode is active. Then ask what hour the cycle starts. A start time in the middle of the night, when no one is running water, keeps the cycle from interrupting showers and laundry. Also confirm the controller clock shows the correct time of day, because a wrong clock means the cycle runs at the wrong hour.
Confirm the salt and the brine level
Ask what kind of salt the unit takes and what level the brine tank should reach. Look inside the tank yourself. Salt should sit above the water, and the water should cover the salt rather than leave it dry at the bottom. Ask how the installer set the brine draw and whether that setting matches the salt type and the manufacturer's instructions.
Label the bypass and the shutoffs
Ask the installer to show you the bypass valve and explain what each position does. Have them mark the position that sends water through the treatment and the position that sends it around. Ask where the main shutoff for the unit is, and where you would close it in an emergency. Ask how the drain line ends and whether it has an air gap, so a regeneration cycle has a clear place to discharge.
For filters, check the backwash and the flow
A sediment or carbon filter with a backwash function needs its cycle set. Ask how often it backwashes, how long each cycle runs, and whether it runs on a timer or on a meter. Ask what the rated flow is and whether your busiest hour stays under it. You can check flow at a nearby fixture yourself by filling a container for a set number of seconds and reading how much came out.
Write the settings down and keep the card
Before the crew leaves, ask for every setting and value on one sheet. Include the hardness reading and the number programmed, the regeneration mode and start hour, the salt type and brine level, the bypass positions, the shutoff location, and the filter backwash interval. Ask for a copy, and a photo of the controller screen if the installer allows it. When a technician comes back later, they can compare the card against the unit instead of guessing what was set.