A softener that wastes water rarely announces itself. The water bill climbs a little each cycle, the house still runs soft, and nobody connects the two. The early sign is in the meter and at the drain, not at the tap.
Read the meter with nothing running
Turn off every faucet, the ice maker line, the dishwasher, and the washing machine. Find the water meter, usually at the street side in a box or in the basement or garage, and write down the reading or the position of the low-flow dial. Wait at least an hour with no use. If the reading has moved, something in the house is drawing water. Your list of suspects is short: a running toilet, a leaking supply line, a hose bib, or the softener's drain path.
Rule out the toilet first
Toilets cause most phantom meter movement. Put a few drops of food coloring in the tank, wait without flushing, and check the bowl. Color in the bowl means the flapper or fill valve is leaking. Fix that before you blame the softener, because a running toilet looks exactly like a softener problem on the bill.
Watch the drain line when the softener should be idle
If the meter still moves with the toilet ruled out, find the softener's drain hose. Note the time, then hold a bucket or watch the outlet for a minute while no water is used. A softener at rest should not send water down the drain. A steady trickle, or a drain that runs for a few minutes at odd hours, points to the control valve. When the valve sits partway between cycles, a small amount of water moves to drain all day. The valve may also be leaking internally past the piston or seals.
Check the regeneration record
Ask the service provider or check the control display for the regeneration schedule and the last few cycle times. Compare what you see with what the installer set. An extra cycle, or a cycle that starts when no water has been used, is worth writing down. Also check the salt level. If the brine tank drops faster than usual with the same household, the softener may be cycling more often than it should.
What to tell the service company
Bring the meter readings, the times you watched the drain, and the result of the toilet dye test. Ask them to confirm whether the control valve is supposed to sit in the service position when idle, whether any drain flow is expected at rest, and what they will check first. Ask whether the repair covers the valve or only the cycle settings. Avoid opening the valve head yourself. A valve that will not hold position is usually a service job.