Add a few drops of food coloring to the tank water without flushing.
Wait about 10–15 minutes, keeping the toilet unused.Check whether colored water has appeared in the bowl.
If the color reaches the bowl without flushing, water may be leaking past the flush valve or flapper.
This test can help identify a possible leak, although it won’t diagnose every plumbing problem.
If the toilet continues to run, inspect the relevant components or contact a plumber.
A simple replacement flapper may be all that’s needed, depending on the toilet
