You've got 5 wires. 3 yellows, a red and a black (or green)
The yellows come from the alternator and the red, black go to the battery via the voltage regulator.
To check each diode in turn, using a multimeter on the ohms range, or the diode range. Or you can use a battery and bulb set up.
The diodes only conduct in one direction, which is what you're looking for.
Put a meter lead on the negative and check each yellow in turn.
Put the other meter lead on the negative and check each yellow
You've now checked D1, D3 & D5.
You should have a reading one way but not the other way
Put a meter lead on the positive (red) and check each yellow.
Put the other meter lead on the positive and check each yellow again.
You've now checked D2, D4 & D6.
If it checks out, good. Put it back on the bike.