Hi Reinhold,

for testing the diodes you should un-mount the generator, but don't have to take it apart completely. Here are a few pictures of the generators internals:

for testing purposes any digital multi meter is suitable, provided a diode test is available. Any instrument will do.

One probe of the instrument goes on the base plate of the diode, the other goes to the pin (wire).
Verify current flow and current block by switching polarity of the probes.

The voltage over the diode in current flow should be approximately 0,45 Volt. If the diode measures flow in both directions or 0 volt over the diode, then the part is considered defective and needs replacing.