How do I test whether a shaker or its power supply is the problem?

The exciters appear as Windows audio devices. Set the suspect pedal as the default audio output and play music:

• Brake plays music, throttle silent (or vice versa) → the silent pedal’s DC adapter has likely died (each pedal has its own 15 V supply). Support ships a new adapter. Power surges/outages are the usual killer — a surge-protected strip is recommended.
• Sounds distorted/rattly → loose coil (see the repair guide above).
• Test at 100% volume with your foot on the pedal — an unloaded pedal always sounds much harsher than it feels in use.