There are two types of "voltmeters" that are typically used in our hobby...

1) loaded voltmeters that provide a simulated load (~250mA) while the voltage is measured
( http://www2.towerhobbies.com/cgi-bin...?&I=LXNK81&P=0 )

2) High impedance or no load voltmeters or "voltage monitors"
( http://www2.towerhobbies.com/cgi-bin...&I=LXHDJ2&P=ML )

The loaded meter is typically connected to your battery via the charger connection on your switch harness. The switch must be in the OFF/Charge position, and the receiver and all the servos are unpowered. Your battery voltage drops under load, so the simulated load built into this type of meter gives you a more realistic measure of the amount of charge left in your battery pack.

The voltage monitor is connected to the receiver via a servo plug, and continuously displays your battery packs voltage while the receiver switch is ON. Since the receiver and servos are powered while this voltage monitor is on, there is already a realistic load on the battery, so no simulated load is required to get a good reading. I usually move both transmitter sticks around rapidly (at the same time) to put a little larger load on the batt, and watch the voltage monitor to see what I get.

Personally I use a Voltwatch battery monitor in all of my planes. I've seen at least two guys loose their planes due to low battery voltage. Both guys had a loaded voltmeter to use, but neither of them used it because they had just taken their plane off of the charger and assumed it was good to go! With a voltage monitor, you always see the batt pack voltage when you turn your plane on... before and after each flight.
Some guys argue the loaded meter more accurately simulates the load you get while flying, than just banging the servos around on the ground, but I don't think that makes much difference. The main thing is the voltage monitors are always on, displaying your batt voltage, but you have to remember to use loaded meter to check your batt, and most guys don't use loaded meters before every flight, it's just a hassle, so they assume the batt is good for a few flights before they check it again!