If you research your filter online, you should find a filter factor for it. Some CPL filters cut down the amount of light reaching the sensor by 1 to 1.5 stops. Others might cut it by 1.5 to 2 stops. The camera's meter will compensate by changing either the ISO, shutter speed, or aperture when the filter is mounted so you won't really notice much, if any, difference when the filter is mounted on the lens.
When you rotate the filter on the lens, the entire scene won't be affected by the CPL filter. Only the surfaces that are affected by polarized light will be visible to those changes. Keep notice of the sky, reflections in water, reflections on glass windows, and the color of the grass and/or leaves in your scene. Those should change slightly in color or appearance providing you are standing in the correct direction in relation to the sun. If you aren't standing and aiming the CPL in the correction direction to the sun, you won't notice any changes when you rotate the filter.