How to diagnose a broken Aprilaire humidifier (Model 600)?
It's likely functioning properly. A setting of 6, probably correlates to 40% relative humidity. According to the Owners Manual


  Your Aprilaire Automatic Humidifier, is a high precision system that will
  accurately maintain the relative humidity in your home to a maximum of
  45% RH and a minimum of 10% RH.


If the home is at a higher relative humidity, the unit should not come on.