Water leak tips for high water bills are always good to hear about. Copper pipe leaks article.
Faucet leaks are obvious costs to the water bill. Even a blind man could feel the water or the continuous wet spot in the sink. When you have a high water bill and you can’t see a leak, it may be in the toilet. You might not hear a leak or even see it.
Go in the bathroom, take the stuff off the top of the toilet tank, remove the lid and place in a safe place. Look in the tank, and check the water level against the overflow tube (its about 10 inches tall and 1 inch around in the middle of the tank).
Is the water level up to the top of the overflow tube? Is it draining into it? If it is, then the ballcock must need repair or replacement (that tall thing on the right with the float attached).
The best advice here is to take the top of the ballcock off and bring it and any lose parts to a hardware or plumbing store. Good stores will have in-store help and parts to repair a ballcock, and new ones to replace the unrepairable.
BUT you say the water is below the top of the overflow tube! Now what? My guess is the water is getting out. Throw some food coloring, available at some City Halls or Water Departments, or even your food pantry. Wait 20 minutes or less.
Is the color in the bowl? Yes? Remove the flapper or rubber ball flap at the bottom/center of the tank and take it with you to the store for replacement. Some contraptions here are delicate so take care when removing.