To manually check if the PWM has a low frequency: put your finger in front of the screen (or the led lamp you want to test), and move it quickly to describe a p

To manually check if the PWM has a low frequency: put your finger in front of the screen (or the led lamp you want to test), and move it quickly to describe a paper fan (V). If you see a stroboscopic effect, the frequency is too low. => The screen is not flicker free or the screen brightness too low. This is only a quick test not a bullet proof solution.