logixoul It depends on the frequency of PWM. Low frequency like 240Hz is easily detected by just the image, as there will be one or more black bands in the image. If it is very high frequency and the time the pixels are off, it is difficult to detect as in a 4k screen there would be just one thin line that is "off".
But yes, best way is to take a 1/4000 shutter speed video on manual settings and actually watch the video on a big screen. I was able to detect my Ring Light flicker that way. It has a PWM in the Khz range and in the video I'm able to see slight banding, which I was not able to see in the DSLR screen.
And of course, the Ring Light produces eye strain if I use it for video calls for say 1-2 hours a day. Just noticed it a year or so ago, when I got it and was wondering why my eyes were strained after using it.
Frustrating that they put PWM into everything and claim it is problem free if the flicker is in the Khz range, because it just is producing eye strain.
My theory is that as the light is ON in full power even for a fraction of a second, the optic nerve recognizes this high power light and the pupils will try to constrict like they would when watching a bright light. After an hour or so, when the pupils are trying to accommodate the bright light by fully constricting, the eyes get strained and red.