Determine voltage of PWM signal with only a multimeter
The LED voltage seems to be on the low side of the driver. This ranges from the Off state of the LEDs (Vcc-VLeds) (&lt;1V?)  and the On state of the FET switch (50 mV??)
So making a TACH circuit for your DMM you can make a crude estimate of the inverse PWM since you are sensing from the bottom side.



simulate this circuit &ndash; Schematic created using CircuitLab