How would you determine if the diode were good or defective?

Firstly, to be absolutely certain, one leg of the diode should be unsoldered and lifted from the Printed Circuit Board (PCB). There are many considerations here:-

The Power must be disconnected from the device, you will need a suitably sized soldering iron, you will need a solder sucker or wick and solder to reinstall.

Some previous soldering experience would also be valuable.

Secondly, you will need a Digital Multimeter (DMM) with a diode test function.

Assuming the above, identify the Anode and the Cathode of the diode as follows:

The Cathode is generally marked with a painted or etched band or stripe.

Using the DMM, switched to the Diode Symbol, red lead plugged into the + terminal, black lead plugged into the - terminal, put the red lead on the Anode and the black lead on the Cathode of the diode.

A good silicon diode should indicate between 0.6 & 0.7V on the display of the DMM. Reverse the lead connections at the diode and the display should read "OL", which means the maximum reverse voltage is applied and no current flows.

An indication of 0V in one or both directions or OL in both directions means a faulty diode.