Find where you can pick up a suitable signal from the index sensor (TP9 according to the schematic in the DDI-1 service manual, or on the photo transistor connections to the drive board) or use the /INDEX connection on the interface cable/connectors (but the drive must be selected by the computer) and use a frequency counter or oscilloscope. Or less good, identify a mark, or make a mark on the rotating part of the drive assembly. Then use a strobe light.
If the disk drive controller has not detected that the drive is rotating within the correct rotation speed, it will return a drive not ready error. See also the drive ready output on the interface cable/connectors, is this at the correct logic level?
I *think* there are other problems that also cause a drive not ready error. This may include faults in the connectors/connecting cable. But I’ve not fiddled with a faulty floppy for some time now on an Amstrad…
Mark