Put your headlight on a towel, lens downwards and throw in the cottonball. Move it around with the chopsticks. If you want to turn the headlight around to see if you cleaned it all remove the cottonball before that.
beginning

while (one half done)

at the end

Actually the old beam of my Paso was even more dirty inside. A good indicator is when you can`t see the bulb inside clearly. A dusty headlight looks quite normal until you start cleaning it and see the difference ...
I recently bought a brandnew headlight for my Vespa scooter and it was the same.
G.