Gale from Toronto, Ontario sent us the wonderful idea of simply putting some food colouring into the toilet tank to spot flapper leaks. If the colour shows up in the bowl below, you have a leaky flapper that needs replacing. Actually that is almost conclusive.
There are two ways that water gets from the tank into the bowl: through the discharge that is controlled by the flapper, and through the overflow. So if the water is not high enough to spill into the overflow tube and the food colouring does show up in the bowl, then the flapper is either not seated correctly, or more likely has become clam shell shaped by exposure to chlorine. Those cleaning pucks that you put into toilet tanks just eat rubber and should never be used unless you understand that you must replace the flapper more often. For a picutre of chlorene attack on a flapper, click here.
In most tanks there is a water level mark on the side of tank, on the inside. If you do not have such a marker you should adjust the water to stop about 3/4 of an inch below the overflow. Adjust this first before trying the food colouring. Then if the colour shows up in the bowl, Gale is right that you need to replace that flapper.