RE: regular voltmeter and onboard?
Ditto the cheap digital multimeter and resistor. No need to buy the expensive Hobbico or Hanger 9 meters in my opinion. The multimeter can also be used to check continuity, measure dc and ac voltage and current. It's much more versatile than the specialty RC meters.
The little on-board flat thing you refer to is probably a VoltWatch. I for one find them very useful, however, they cannot be used in the same manner as a loaded meter. You need to wiggle the servos and see how the monitor responds. It actually can give you more information than the loaded meter, but again, you must learn how to interpret it.