If you can get your hands on a servo just for the test that would be the easiest way. If the issue is still there with a different servo then the BEC would have to be the culprit.

If you have a multimeter you could check the voltage coming from the ESC to the receiver and see what it does when you turn the servo from left to right. If it fluctuates wildly then it's the BEC guaranteed and is not then it is the servo.