SECOND: Put several drops of food coloring into the water tank above the toilet (not in the toilet bowl :)), be sure to put enough dye into the tank that you can see the color change in the tank.
THIRD: Wait 15 minutes WITHOUT flushing the toilet. After 15 minutes, if the toilet bowl water is the same color as the tank water, you can assume you have a leak.
If there is a leaking toilet, more than likely you can fix it by replacing the flapper valve, an inexpensive toilet repair you can probably do yourself.
This simple test works great and it saves money by stopping the costly water leaking into your toilet.