A toilet flapper seals water in the toilet tank until it's time to flush. Over time, however, the flapper can get worn and start leaking. There are a few easy ways to determine if your toilet's flapper needs to be replaced.
Listen for a constant, slow trickle -- you may hear the toilet running if the flapper isn't sealed completely. If there is a leak, you also might notice a slight increase in your water bill.
Remove the tank lid and lay it on a towel on the floor so it won't get broken. Place a few drops of food coloring into your toilet tank. Don't flush the toilet for 15 minutes. Check the toilet bowl -- if any of the food coloring appears in the bowl, the flapper is leaking.
Inspect the toilet flapper. Turn off the valve that supplies water to the toilet -- it's usually found behind the toilet. Remove the tank lid and set it safely aside. Reach in the tank and pull the flapper up out of the water by its chain or plastic arms. Look for bumps and dirty or eroded areas on the bottom of the flapper -- if the surface is uneven, the flapper won't be able to lay flat and create a tight seal. You can try cleaning the bottom of the flapper, then turn the water back on to see if that solves the leak.