There are several main problems that occur with toilets. Most of the moving parts on a toilet are inside the toilet tank. These include the flush valve, the flapper and the handle assembly. Problems can occur with the wax seal between the toilet base and the flange in the floor. The water supply hose that connects the flush valve with the shut-off valve can also wear out and leak. Diagnosing most plumbing problems with toilets is accomplished in just a few minutes.
Dry the area around the base of the toilet with paper towel if you think water may be seeping from there. Place dry paper towel at the base of the toilet and check it in a couple of hours. If there are wet spots on the paper towel, the wax ring may have worn out and need replaced.
Wipe off condensation from the bottom of the tank and the water supply hose if you suspect a leak there. Use paper towel to dry the area as much as possible. Get down at floor level and watch the supply hose and the fittings at both ends for drips. Normally leaks in this area are caused by a worn out water supply hose that needs replacing. If the water appears to be coming from the bottom of the tank, the flush valve may need replacing.
Remove the toilet tank lid and flush the toilet. If the toilet tank seems to fill for a long time watch the flapper and the flush valve. If the flapper is not making a good seal with the hole in the bottom of the tank, it may need replaced. If the float on the flush valve sticks or if you hear the water just trickle into the tank, the flush valve could be worn and need to be replaced.
Check the length of the chain that connects the handle to the flapper and ensure that it is long enough so the flapper sets in the hole in the bottom of the tank. Ensure that the chain is not too long or it could tangle and cause the flapper to not drop down far enough.