You can, But. What you are interested is not the static voltage, IE unloaded voltage but the loaded voltage. What you want to measure is the voltage under load of the receiver and servos moving, or the transmitter in operation.

Almost all transmitters have a voltage reading, either a digital voltage or on the older transmitters an analog meter showing "output". If your transmitter has the digital readout, you have a good measurement of the actual voltage of the pack under the load of the transmitter. THe "output" meters indicate the condition of the pack in that if the voltage drops, it will be indicated on the meter. Not an absolute measurement.

For the receiver, you have two options. First is to use an expanded range voltmeter. These come in two types, a loaded and unloaded. You wan't the loaded type because you are usually taking a reading of the battery from the charging jack. A loaded meter will give a truer reading of the pack. The second option is a device like the VoltWatch. It functions as an expanded range voltmeter but it reads the voltage and traslates them to LEDs. One led will light indicating the voltage at that point. The great thing about the voltwatch is that it is reading the voltage from the distribution bus in the receiver, IE where the servos plug into. It eliminates all variables like switches and shows just what the receiver is seeing.

You can, with a little Ohms Law put a resistor across the leads of a standard volt meter and get a loaded reading. You would want a resistor that would cause a 500ma draw and would handel a 3w or better load.