This may be old news to some, but if you own a digital multimeter it comes in handy for testing the drain on your batteries under flight loads to see if they are up to the task. Just make a lead for your meter to plug into a servo extension, which will give you the actual voltage your receiver is getting. Here are the checks you can do with the tool.
1. Check every control surface through its full movement to see if it's binding. Do this one regularly to find faulty servos or wiring.
2. Manually stall one or more servos to see how much voltage you get under maximum flight load.
3. Cycle after test two fails to see how much useable capacity you have.

Since the stall test is repeatable, it is just as good as using a loaded ESV. This tester also tells you if you are using good enough batteries for your application. If the voltage falls too low but your cycler says you still have say 2/3 capacity in your batteries, then you are putting too much draw on your batteries. Switching to a higher discharge rating or higher capacity will give you better battery life and many more flights per charge. The multimeter also comes in handy for checking the impedance of wire connections and solder joints, and can check the current draw of servos to spot potential problems before they cost your an airplane.

I use the amp setting in the multimeter for checking servo movements, as you will see if one is sticking or binding anywhere in the servo travel as a fully charged battery will hold voltage on a miner binding condition, even a stallet servo won't change the voltage much on a fully charged battery but will show up easyly in the amp draw. I use the 500 milliamp setting.

Cheers

Good tip, The Ghost.

So are you saying that you plug the extension into any open receiver port and then exercise your servos to get a reading?

Or do you hook the extension in line to a particular servo with a y cable ?

As you can see, I'm obviously confused.

A Voltwatch installed in your plane does nearly as good a job of detecting servo stalling, push rod friction, poor switch etc. plus it is always there and you can check prior to and after each flight with no added equipment to have to find and plug in. If you see those LED's dance up and down the strip when you cycle the sticks, you know you have a problem. The nice thing about the Voltwatch, it checks much more than just the battery condition under a static load.

Yes, I plug it into a spare port using an aileron extension. I have used the voltwatches before and they do work, but I like this better because I get actual voltage numbers instead of lights.