A leaking toilet bowl can be one of the largest wastes of water in a house. Toilet bowls will invisibly and silently pour more water down the drain than anything else in the house. The internal parts of a toilet are under constant attack by the water and the chemicals used to sanitize the water. Chlorine and chloramines cause the rubber to deteriorate over time. The average life expectancy of the rubber components is about 5 years. This life may be reduced even more if in-tank bowl cleaners are used. Most in-tank bowl cleaners have chlorine bleach as a major ingredient and increase the level of chlorine in the tank well beyond what rubber can tolerate, leading to a service life of even less than 5 years. Toilet manufacturers will void the warranty of a toilet that has had in-tank bowl cleaners used in the toilet.
The first thing to check is inside the toilet tank. Remove the tank lid and set it on the floor. Tank lids can break easily and be hard to replace especially on older toilets putting the lid on the floor makes sure that it will not fall and break. Make sure the water level in the tank is at the proper level. It should be either to the water line marked on the tank, 5/8″ below the top of the overflow tube or, the line on the overflow tube. If the water is spilling over the top of the overflow tube, stop right now as you have found a big leak. Try adjusting the ballcock (fill valve) so the water level is maintained at the proper level. If the ballcock does not properly maintain the water level in the tank after being adjusted then it will need to be replaced. This is the silent leak that uses huge amounts of water.
The next two problems discussed usually result in what us plumbers call a ghost flush where you periodically hear the ballcock turn on and refill the tank even though no one has used the toilet. These are probably less dangerous in terms of wasting water because someone will hear the noise and usually take action to repair the problem.
The next thing to check is the tube that runs from the ballcock to the overflow tube. This tube is what refills the bowl with water after flushing. If this is shoved down into the overflow tube too far where it ends up being below the water level of the tank it will siphon water from the tank into the bowl where it then goes down the drain.
There are dye tablets available to test toilets. These are frequently available at plumbing supply houses, hardware stores, and Home centers. However, they are not required for the dye test as food coloring may be used. Put either the dye tablet or, food coloring into the tank. Do not flush the toilet! Wait 15 minutes then check back and see if the water in the bowl of the toilet has changed color. If the water has changed color the flapper should be replaced, then the toilet retested with the dye. In most cases the flapper replacement should fix the problem. Some times it may also be a case of the flapper chain being too tight or, a trip lever moving causing the chain to become tight as the lever moves.