For this you'll need a multimeter that can measure resistance.
I followed the advice in this link (
http://ezinearticles.com/?Mercedes-Benz-Alternator-Parts-Replacement&id=345576) under the section “TESTING & REPAIR”.
QUOTE:-
Diode Assembly
1. Place ohmmeter scale on x100 scale connect ohmmeter leads across "B+" terminal and 3 stator terminals one at a time, reverse leads, ohmmeter should indicate continuity in one direction only.
2. Reconnect ohmmeter leads across negative and 3 stator terminals one at a time.
Reverse leads; ohmmeter should indicate continuity in one direction only.
3. Connect ohmmeter leads across "D+" terminal and 3 stator terminals one at a time.
Reverse leads, ohmmeter should indicate continuity in one direction only, if diodes are found to be defective, replace the diode assembly.
Fig. 3: Diode Assembly Test Diode assembly for 55 amp alternator is shown, others are similar.
1. Place ohmmeter on lowest scale, connect ohmmeter across stator leads, resistance between leads should be 14 15 ohms for 55-amp alternator and .09 10 ohms for 65 through 90 amp alternators, if resistance is incorrect, stator has open or shorted windings and must be replaced.
2. Place ohmmeter on X 1000 scale. Connect ohmmeter between stator core and stator lead, no continuity should exist, if continuity exists, stator is grounded and must be replaced.
1. Place ohmmeter on lowest scale, connect ohmmeter across slip rings, resistance should be 3 4-3.75 ohms for 55 amp alternator and 2.8 3.1 ohms for 65 through 90 amp alternators.
2. If resistance is too low, rotor has short circuit and it must be replaced if resistance is infinity no continuity, rotor has open circuit and must be replaced.
3. Place ohmmeter on x1000 scale, reconnect ohmmeter between either the slip ring and rotor core, no continuity should exist, if continuity exists, rotor is grounded and must be replaced.
END QUOTE
To be honest the diode testing went well. Then, when I got past where it says “Fig. 3:...” I got nonsensical readings from my multimeter. I'm not 100% certain that the numbers quoted in this article are correct. The authors conversion from lbs-ft to Nm at the end of the article is incorrect. Anyway if anyone else follows this advice I'd be interested to hear how it went.
As my alternator was working fine before I took it apart, I guessed everything was still OK so I carried on regardless – as I'm sure you can tell I'm not really electrically minded – I don't have the patience for those bloody multimeters!