Check the position on analog servo
It appears that the published specifications for the sm-s4315r are poor. Sending a 1.5 ms control pulse tells the servo to stop.

It appears that normal operation assumes a continuous pulse train of about 50 pulses per second. Pulses shorter than 1.5 ms result in continuous rotation in the clockwise direction. Pulses longer than 1.5 ms result in counterclockwise rotation. Speed is inversely proportional to pulse width for the clockwise direction and directly proportional to pulse width in the counterclockwise direction. Supply-voltage change within a specified range also has an effect on speed.

The effect of applying power without sending control pulses is undefined. It seems possible that the result may be rotation at maximum speed in the clockwise direction.

It seems unlikely that there is any way to verify the speed, direction of rotation or position without adding shaft speed/position sensor.

There seems to be a "rest point" position adjustment, but I didn't' find a clear explanation of its function. A comment has convinced me that that this device has no positioning capability and that there is no way to "reset" or make it return to a specific position. If that is a requirement, a different servo or an added position sensor and external control programming is needed.