mfr02

You can buy a servo tester, or Google for servo tester circuits and build one. However, you have done half of the practical testing already, by substituting a different one. You did say that it worked at first, so the only other practical test is to try it in another slot. If the problem moves with the servo, the problem probably is the servo.
Some brands of servo had different connection schemes, and some have different throw responces, and many manufacturers insist on having different shaped plugs, but they basically all respond similarly to the same control signals.