Back there you were told to "check with the power off". I assume he meant for you to turn the servo arm manually nd look for normal movement.
NOT RECOMMENDED. There is no standard, but there is a very high gear ratio between the motor and the output gear, which places high loads on the gear's teeth. Obviously, this is more of a problem on plastic gear equipped servos.
Reversing the operation multiplies the gear ratio, and greatly increases the force on the gear's teeth, sometimes breaking some or distorting them enough to cause erratic operation.
Forcing the servo into reverse operation will not prove anything anyway. The gear train is working in reverse, which is not the way it rotates normally, the way it is intended to.
Otherwise, the advise to replace if in doubt is good.