Unplug and remove the diode plug. With a digital multimeter, set it to the diode function. It usually has a schematic diagram of a diode on it. Connect the red (positive) lead of the meter to the side of the diode that has a band, and the black to the other end, you should get a reading, it should be somewhere around .4000 to .7000. Now reverse the leads, it should not have any reading at all, OL? (open line). What you are measuring is the voltage drop across the diode as it goes into forward bias. Usually around half a volt. (.5) When you reverse the leads, you are “reverse biasing” the diode and there should not be any current flowing at that time.
Its important that you use the diode function on a DMM, as it sends enough voltage to bias the diode. If you just use the “continuity check” (ohms) on a DMM, it may not send enough voltage to bias the diode and it will look like it is shorted both ways.
With an analog multimeter, they usually send enough voltage to bias, so with one of those, on the Ohm function at its highest setting, it should conduct one way, but not the other way.
Does the GEN2 light come on now when the key is on but vehicle not running?