PWM is caused by the monitor only.
3 ways to detect: Low frequency - just whisk your open hand between your eyes and the monitor back and forth quickly - do you see "many borders in your fingers, or like sawtooth"?
Bring the camera in touch with the screen when there is a white page like this page. Your smartphone will automatically adjust to the high amount of light and adjust the shutter speed to the fastest possible, you should see flicker. Some phones thought compensate the flicker so you have to take a picture. Do you see dark bands in the picture?
Select shutter speed of 1/4000 and record video or take a picture. Do you see black bands in the picture or flicker in the video?