I favor the VoltWatch over the loaded DMM/ESV technique. Significant current flows in the leads to the loaded ESV, causing some voltage drop across the leads and connectors. This can present pessimistic voltage readings. For example, the meter might read 4.7V while the voltage at the battery is actually 4.8.

I check my voltwatch immediately after a flight, before I turn off the receiver. This avoids the problem of battery recovery, where the battery voltage tends to gradually climb back up after the load is removed only to fall rapidly again once the load is reconnected (next flight!).

One note of caution on the voltwatch. I tested about 5 against a trusted Tektronix DMM. I found that they vary about 0.05 volt from unit to unit. It would be wise to test your VoltWatch before trusting your airplane to it. Same holds true for ESVs. These things are inexpensive and quailty control may not be the best.

Another benefit of the Voltwatch is that you can tell if you have a stuck or malfunction servo by moving the sticks and watching how the LEDs respond.

By the way, there are several other products similar to the voltwatch. One is a kit that was written up in Model Aviation a few years ago. Someone also makes a single LED version that changes color/blinks at the go/nogo voltage. They say it is so bright that you can see it while flying.

No matter what technique you use, judging battery capacity from voltage readings is a tricky business. You need to know the discharge characteristics of your particular battery pack and understand a little about electricity.

Good luck.
Jon