A toilet tank leak can not only cause the water to run constantly in the bathroom, but it can cause some serious damage to the floor and surrounding walls and cupboards. Troubleshooting the problem is the first step in fixing the problem, and this can be done within a few minutes by using some dry rags and a flashlight.
Step 1: Dry the Tank
Before trying to find where the leak is coming from, dry the entire tank off with dry towels. Use the flashlight while doing this to watch for any instant leaks that may appear. If they do, you have isolated the problem.
Step 2: Check the Plumbing Lines
Before assuming that the toilet tank is cracked be sure to check all of the plumbing lines. Tighten them with a wrench and check for any cracks in the ends of them. One of the most common causes for water leaks around the toilet tank is from loose or broken plumbing lines.
Step 3: Check the Seals
Run your hand along the outer and inner seams and seals where the bolts, flush valve, and the tank itself. If the water is coming from one of these areas, that particular area will have to be repaired or replaced.
Step 4: Check for Cracks
If the leak can still not be isolated then run your hand over the entire surface of the tank because there is probably a crack within it. Isolate where the crack is and decide if it is a at a fixable location, or whether the toilet tank will need to be replaced.