A continuity check will do but the voltage drop across the internal resistance of a wire is very small. What you detect, if you had a problem, might be caused by corrosion at the connections between the extension and anything it is connected to. At any rate, the resistance is very small and hard to detect.

If a problem is suspect, your advise is sound.. replace the extension.

The problem is that our extensions are measured in inches not feet. For instance a 12 inch extension will have such a small internal resistance, that it might be very difficult to read. And, if it had a high resistance, the problem would be evident.. the servo would most likely not work at all and swapping it out would fix the problem.

CGr.