This should be tested using a variable DC power supply, connected as displayed in the image below:
Once connected, begin at 0VDC and slowly increase voltage until the servo begins to respond. That will be your "start-up voltage."
DFC90 Servo Start Up Voltages
Roll Servo should be less than 3VDC
Pitch Servo should be less than 3VDC
Pitch Trim Motor should be less than 5VDC