Depending on the type of fill valve you have depends alot on how water canbe wasted.
1- You can try puting either a dye in the tank or instant coffee and look lok intio the bowl to see if you can spot a color change
2- Look down the tanks refill tube
(a tube about 1″ in dia) and see if any water is by passing the stopper/ tanb bulb / flapper
You can also check the ball cock or fluid master type controllers to makesure the water level is not so high asto go down the over flow tube attached to the douglas valve
Fluid Master now makes a new type of controller that will not waste water like before from a defective tank bulb
SylvanLMP