To test the flapper plug, remove the lid from the toilet tank and add food coloring to the tank. Wait 15 minutes or so. If the coloring appears in the bowl, the flapper plug is leaking and should be repaired or replaced. If the water level is at the overflow tube the leak may be occurring at the float shutoff valve. Toilet tank leaks typically result from worn parts or misalignment or some part of the flushing mechanism.
Faucets are the other major source of water leaks.
Most drips can be fixed by replacing the o-ring or packing washer inside the valve. Water leaks are costly. A slow drip can waste 15-20 gallons per day. It is not unusual for a unseen toilet leak to waste 100 or more gallons per day. Besides saving water, fixing the leaks will also save you money on your water and waste water bill.