Stand at the toilet when nobody has used the water. Listen for a short hiss or a soft trickle from inside the tank. If the water level drops and the fill valve comes on by itself, the tank is losing water into the bowl. That is the early change. It is usually small, and it is easy to miss because the bowl water looks normal.
Write it down. Note the day, the time, and whether anyone ran water nearby. A pattern that repeats on its own is worth more than one sound you happened to hear.
Run the dye check
Put a few drops of food coloring in the tank, not the bowl. Do not flush. Wait about fifteen minutes, then look at the bowl. If color shows up there, the flapper or its seal is letting water pass. If the bowl stays clear, check the fill valve next.
Sort the parts
The flapper sits over the drain opening and is the most common path for a slow leak. The fill valve controls the refill, and it can hiss or keep running when it does not shut off fully. Both are small parts. Before you buy either one, match it to the model stamped inside the tank, because fittings and heights vary.
Check the meter
Read the water meter at a quiet time when no fixture is in use. Read it again after a set number of hours with the house still idle. If the dial moves, water is passing somewhere. This check tells you the leak is real even when the bowl looks fine.
What to ask before you call anyone
Ask whether the fault is in the flapper or the fill valve, and ask them to show you the dye result. Ask what the visit covers in writing, including labor and any parts they replace. If a service plan is involved, read its terms before the visit. The contract wording decides what is covered, so read it before you assume anything. Our tables list published figures for typical service life, which is a separate question from what a given contract includes.
When the pattern changes
Keep the log going for a few weeks. If the phantom refills become more frequent, or the bowl stays damp between uses, the seal is wearing. That is a planning signal. Replace the part on your own schedule, before the leak reaches the floor or the cabinet below it.