If you have an electrical meter you can check the voltage and amperage going into the servos at each juncture point. That will let you see if it's a battery problem, an extension problem, or a servo problem. As for checking to see if a battery is up to the task, you can hold two control surfaces and move them to full stalling, then look at the voltwatch to see if the voltage your system has is good enough. That's the worst flight load you'll ever have so if the plane passes that test, it'll fly.