I use a small portable fan, shine the flashlight through it and look at the fan blades.
I'm not sure what's the rpm (revolutions per minute speed) of the small portable fan I'm using, but it's so far able to show "slower PWM" and "faster PWM" by visually looking at the light passing thru (reflecting thru?) the blades.
For instance, I made a simple video.
one uses Biscotti (faster PWM, not so visible to most people in normal situations)
one uses Convoy 5/3-mode group driver (PWM is a bit more visible)
Left side shows the faster PWM on Biscotti, right side shows the slower PWM on the Convoy 5/3-mode driver
