Actually, the 3-phase rectifier has 6 diodes. Testing has been covered here a lot of times but I guess once more won't overload the server.
There are 3 A/C input yellow wires, one positive output red wire, and a ground green wire (or mounting stud/bolt). You need a test meter - analog (moving needle) ohmmeter on the lowest ohms range, a digital multimeter meter with a diode test function, or a test light using 6VDC or more. Digital ohmmeters are not good for diode testing, you need that diode test function to use a digital meter.
Each diode is a sort of one way valve for electricity.
Each yellow wire has one diode connected pointing to the red wire, and one diode pointing from the ground. With three yellow wires that's 6 diodes total inside the casing.
Worrying about which way they point is not worthwhile, no diode will ever fail by reversing its polarity - and meters vary in their polarity so you can't tell diode direction by which color probe wire is on which end of the diode without checking your meter on a marked diode.
You can do a functional test which is all that's required.
Each yellow wire is tested to both the red wire and to the green ground wire (or stud). With your meter or test light you should have a positive reading (needle deflection, diode test voltage around .6V, or the test light on) with the test leads connected one way around, and no reading (no needle deflection, "ERR", "OPEN" or "INF" on a diode test meter, or light off) with the test leads connected the opposite polarity. That is 12 individual tests: both polarities from yellow to red and both polarities from yellow to ground.
There are two types of diode failure: open and short. Open will give no reading either polarity, short will give a positive reading both polarities (except a diode test meter will show zero volts).
All six diodes must test OK. One bad diode will reduce alternator output. Several bad diodes will probable give you zero alternator output.
