RE: regular voltmeter and onboard?
Here is my kluge loaded DMM. The DMM is a cheapie from Harbor Freight. I checked it against my Fluke and its DC voltage is right on. The load is an 18 ohm 2 watt resistor. This gives a current of about 270 mA on a 4.8 V rx pack. The Futaba J connector is from Radial RC and comes with banana plugs that plug into the DMM. The resistor is connected in parallel with the Futaba connector using stackable banana plugs, although it could just as well be soldered directly to the connector's plugs.
Desertpig has a good point regarding the active load. With the resistor the load current is proportional to voltage, so, for example the current would be about 540 mA with a transmitter pack using the same resistor. Transmitters usually have built in meters, so this is not an issue for most of us.
One can make an active load using an LM317 IC (Radio Shack carries them) hooked up as a current regulator. It could be mounted to a small aluminum bar to provide some heat sinking and used in place of the 18 ohm resistor. But unless you want to start a new hobby, that would be a silly use of your time.