If you need a new toilet flapper you can easily waste a couple of hundred gallons of water every day. The signs of needing a new flapper often comes in the form of hearing the toilet tank refill when no one has flushed the toilet.
If your flapper is five years old or, older chances are it needs replacement. The rubber in the flapper will deteriorate over time and exposure to chemicals in the water, which make the water safe to drink. The use of in tank bowl cleaners can hasten the deterioration with massive doses of chlorine into the water in the tank. If you use in tank bowl cleaners you should consider using the Fluidmaster Flush N Sparkle system, which bypasses the tank and goes directly to the bowl.
Initially you should remove the toilet tank lid and place it on the floor where it cannot fall and break. Toilet tank lids can be hard to find and quite expensive as well. Look at the overflow tube and make sure the water level is below the top of the tube and water is not spilling over the top. If water is spilling over the top of the overflow tube the fill valve either needs adjustment or, has failed and needs replacement. Make sure the flapper chain is loose and is allowing the flapper to close tight.
To test your flapper for leakage you can use either food coloring with several drops added to the tank or put dye tablets into the tank. Watch for the color from the tank to appear in the toilet bowl without the toilet being flushed. Let it sit for about 30 minutes and see if the color appears. If the color appears in the bowl without the toilet being flushed in most cases the flapper is leaking and should be replaced.
Replacing a flapper is pretty easy and in many cases a generic flapper may be used. Some toilets require a special flapper which you can find in many cases by shutting off the water supply to the toilet and then removing the flapper to take with you to the hardware store, home improvement center or, a plumbing supply house where the clerk may identify the flapper and give you the right replacement. Putting the new flapper in is often as simple as snapping the flapper in place and hooking up the chain with a slight amount of slack in the chain pretty much the reverse of what you did to remove the flapper.