There are 6 diodes inside the 3-phase bridge rectifier and each needs to be tested in both directions.
One end of each internal diode goes to either the positive (red) or negative (green) output wires.
The other end of each diode goes to a yellow wire.
Each yellow wire connects to two diodes, one going to red and one going to green: the diode "directions" (loose diodes all have a stripe, this indicates end to connect for positive output) are such that when the yellow wire voltage is above the red wire voltage, power will flow to red; the other diode lets power flow to green when the yellow wire voltage is below the green wire voltage.
3 phase power is a bit hard to visualize but the voltages between yellow wires cycles up and down as the rotor spins. Power may be drawn from any of the three to either or both of the other two which changes as the voltage cycles from the rotor magnetism spinning in the stator.
So you have 12 diode tests to do. Put one meter lead on red and test Ohms or use diode test to each of the yellows. All three should read the same: either low Ohms (or a typical diode voltage (around 0.5V)) or high/infinite ohms (or whatever your meter displays with leads loose in diode test mode) - and all three readings must be the same (diode test voltage can vary a bit). Then change the meter leads, put the other one on red. Repeat the 3 tests. All must again be the same but the reading must be opposite from the previous test.
Now do it all over with the meter lead on green.
Bad readings: if a diode reads high ohms or "no diode" the rectifier will lose about 1/3 of its output power (this is a guess and it may be 1/6 but still a drop for sure). Still usable for getting some alternator power in an emergency or while awaiting a replacement. If a diode reads low ohms or 0V do not use the rectifier, this causes high stator coil current and may overheat and damage it.
Note that there is no magic in the stock rectifier. Mouser, Digikey, Ebay, whatever, all have monolithic 3 phase bridges that will work super well. At a fraction of the cost for an OEM one. You just have to wire it up: yellow wires go to the "~" terminals in any order... + to red and - to green. You want something like 25A @ 100V PIV - over spec but safer than one with lower specs.
You can even wire up your own with 6 loose diodes, but that's a pain to do and a pain to cool. The monolithic one should have a cooling solution, I've just clipped the wires off a burned out stock one (to use hooking up the new one) then drilled and tapped holes in it to screw down the monolithic replacement. Pretty ugly but I didn't have any other heat sink. Just a chunk of 1/4" aluminum plate cut to fit where the original rectifier was works well. It doesn't make enough heat to need a finned heatsink, but it should have something.