In most situations, you can hear or see your toilet running or cycling, but sometimes you cannot see or hear your toilet running.
There are a few other ways to conclude if you have a slow leak. Check to confirm that water is not spilling over into the tank via the overflow pipe. Another way to determine if your toilet is running is to put a couple of drops of food coloring into the tank, then remove some of the coloured water and reserve. If within 20 minutes the water in the bowl is lighter than the reserved water, you will know that water is leaking and that either the fil valve and ball or flapper needs to be replaced.