[ischmitz]

When the points close or when you short the points with a screwdriver or short the trigger wire directly to GND the box should release a spark.

And yes, please be very careful as there can be high voltage at the coil output once you disconnect the coil primary while the box is whining. Even with the coil connected (and internally broken) there can be up to 400V present. The coil primary essentially serves as path to GND to charge up the internal main capacitor. Once the spark is about to be released the other side of that internal capacitor is momentarily connected to GND and this makes the voltage of the capacitor appear at the CDI box output. At that point the primary "sees' up to 450V and the secondary of the coil transforms this pulse into the HV pulse that generates the spark. Under no circumstances touch the output from the CDI box while it is powered (whining)

Last, you are correct. Even if a box makes the whining sound it isn't proof that it is fully functional. It can still have an issue internally.

Ingo