Turn the valve off that supplies water to the johnny. Put some food
coloring or other dye in the water closet of the johnny, and let it set
overnight. If there is coloration in the basin the next morning, then the
flapper is leaking and must be replaced. Sometimes they look perfectly
fine, but still leak a bit.

You can get a good flapper at Home Depot or Lowes for $ 6-7.

James