A toilet that runs quietly is the most common thing we find, and the one that most often goes unnoticed on a water bill.
Hard water is hard on the parts
The mineral content in the water here wears out flappers and fill valves faster than softer water would. The rubber stiffens, stops sealing properly, and the tank keeps refilling. It is an inexpensive part and a quick visit, and worth sorting as soon as you notice it.
Whole neighborhoods, same fittings
Homes here went up with the same specification throughout, so toilets tend to need attention around the same time. If yours is original and has needed parts more than once, replacing it is often better value than a third repair.
If the toilet rocks
Any movement means the seal underneath has failed, and water is getting into the floor rather than down the drain. On slab construction that shows up as damage to the flooring around the base, so it is worth dealing with rather than living with the wobble.
Rental turnover
If you let a property here, replacing an aging toilet between tenants costs a fraction of an emergency call with someone living there and water on the floor.