A DVM is very useful for monitoring the voltage of the battery packs. If you are having trouble lifting off, then the first thing you should check is the battery voltage while the heli is powered up.
A wattmeter is a device which is connected between the battery and ESC of a helicopter, and measures the voltage, current, the instantaneous watts used, the cumulative battery capacity used, and other parameters. This is very useful to check if the current draw is too high or the battery voltage is dropping too low.
The original wattmeter is the Astroflight Super Whattmeter. This device can handle up to 60 volts and 70 amps of discharge.
A recently released wattmeter is the Medusa Products Power Analyzer II. This is available in three different versions and is available with a USB interface for connection to a computer.
Another recent wattmeter is the RC Electronics "Watt's Up" power analyzer. This appears to be similar to the Astroflight Whattmeter.
Note that a Wattmeter only measures AVERAGE current. This is a slight problem because it does not measure the maximum current draw of a pulse-width modulated motor.