The choice is a voltwatch, which stays in the plane, or a voltmeter which you plug into the charging jack on the switch harness. A meter such as this one has built in resistance to simulate the receiver and servos: http://www.towerhobbies.com/products/towp1181.html
You will need to add leads to this meter so you can plug it into the charging jack. You can buy the premade ones, or make your own. I place more trust in a meter than relying on some lights. It's also cheaper to buy one meter which works for all your planes than multiple voltwatches.

If you buy a switch/charge jack mount, such as this one: http://www2.towerhobbies.com/cgi-bin...?&I=LXJ778&P=0 the jack can be mounted in a convenient place for charging and testing the voltage.