As far as the inside of a toilet goes, the flapper is truly an unsung hero. The flapper at the bottom of the toilet tank prevents water from running from the tank into the bowl. When a flapper malfunctions, the toilet will have loud flushes and continuously running water, and cause huge water bills. Luckily, it is both easy and inexpensive to check and replace the toilet flapper and associated parts.
Remove the lid from the tank and set it aside.
Empty the water from the toilet tank. You can do this by lifting the floating ball so it can't drop. With the ball held up, flush the toilet and it will stay empty.
Look at the arm on the back of the handle. The arm is a solid piece that connects the toilet's handle to the chain. If it appears bent or broken, it may prevent the flapper from sitting properly at the bottom of the toilet.
Check the chain that connects the arm to the flapper. If the chain is twisted or caught on something then it won't let the flapper all the way down. If any of the links are cracked or rusty then the chain needs a replacement
Inspect the flapper. Check for cracks or breaks, and ensure that it is connected securely to the bottom of the tank. You can do this examination by touch but if you need to look at the flapper more closely they typically peel off easily.
Run your finger along the gasket the flapper sits on. The gasket is made of rubber, and sitting in a toilet for years can cause the material to break down. If you see a crack or feel any rough spots, the gasket may need replacing.
Replace any part than seems worn or broken. Any of the parts, from the arm to the gasket, can cause a flapper to leak.