Turn the stroboscope on and turn the rate up until your eyes see a continuous light.  This should be somewhere around 3000 cps.  If you now decrease the intensity of the light with the crossed Polarizers you should be able to turn down the frequency to 1500 cps and see a continuous light.  The more light you block with the Polarizers the lower the frequency you should need to see a continuous light.  In other words the more intense the strobed light the higher the frequency you need to see a continuous light.