u get one of them meters you buy from shop and do it....maybe $20,
or
you have to calculate what is the fundamental sine amplitude in your supply, and its phase, and get it like that
do u have a specific pfc front end or just a fwb?
...if just a fwb then good luck, the calculations are not terribly easy......you will need your ac mains voltage waveform, and your mains input current waveform. -together , so you can see the phase too.
are you ready for the serious mathematical jamboree that you are now about to embark on?
take a look at your current waveform.......it is not sinusoidal is it.
..but you now have to cALCULATE WHAT IS THE "FUNDAMENATAL" SINE OF THAT WAVEFORM......(SORRY ABOUT CAps)
When you have that sine, and have both its amplitude and phase relative to the voltage...then you do Vrms * Irms * cos (phi)
where phi is the phase from volt to current.
that is your "ACTIVE POWER" input to the led driver.
You now have to find the APPARENT POWER input.
That is equal to Vrms*Irms and doea not take account of the phase.
then real power/apparent power is your power factor.
if you want, you can just estimate your active power by knowing what power is consumed by the LEDs, and then divide this by 0.8....assuming 80% efficiency.