If you shut off the water supply to the toilet and let it sit say overnight, if the tank is empty or down, then the flapper valve is leaking and just go to a plumbing supply or Lowes and buy a new one. Another thing you can do is put some food coloring in the tank. If it's clear in the morning or there's color in the bowl, it is leaking. Depending on your water, they will last anywhere from 6months to many years. Never use any chemical cleaners in the tank. If there is any crud on the seat for the flapper valve, it could leak. Rub your finger around the seat and see if it is nice and smooth.
If the chain is too long or short, it could either get caught up underneath the flapper, or hold it up slightly.
If the overflow filler hose is not in its clip, it can siphon and cause problems.