There are a number of laptops with high frequency PWM, which is above 10khz. At these frequencies, the "normal" method of checking PWM, which is to look at the screen from a smartphone camera, would not work. A smartphone normally can only detect PWM if the PWM frequency is around 200Hz. So if a laptop have, say 20kHz PWM frequency, looking at the display from a smartphone camera will appear as if the display has no PWM, when in fact it does.
For really high frequencies, you would need an oscilloscope to detect it, like this Lenovo Thinkpad 13 laptop:
Some review sites say PWM with higher frequencies (i.e. above 10kHz) should not give eye strain, but I am not so sure. Personally I have never used a laptop with high PWM frequencies, so I would not know if I get eye strain on it or not.