Tower Hobbies, or most reputable LHS sell an Expanded Scale Voltmeter (ESV). It is designed to put a slight load on your batts when it checks them. Otherwise, you only know what the static voltage is, and if your electrical gear, RX, Servo, accessories, puts a load on them, a good battery on the ground can become a bad battery during use. Also great for calculating how many flights you can get out of a battery, both TX and RX.

Im sure someone has a link for the Tower Hobbies, or Hobbico ESV.