Depressurize your home to help detect leaks. On a cool, very windy day, turn off the furnace. Shut all windows and doors. Turn on all fans that blow air outside, such as bathroom fans or stove vents. Then light an incense stick and pass it around the edges of common leak sites. Wherever the smoke is sucked out of or blown into the room, there's a draft.
If you suspect your toilet is leaking, but aren’t sure where, add some food coloring to the water in the tank. If you see colored water inside the toilet bowl, suspect a defective flapper that needs to be adjusted or replaced. If it shows outside the toilet near the back, suspect a faulty tank-bowl gasket. If you see any water on the floor near the base, check for a faulty wax ring.