I diagnosed mine by turning off the battery charger and discharging the batteries to some degree. Then, when I started the engine, I checked to see if the alternator was supplying any voltage. It should jump up to the 13.5 - 14.4V range. I initially used my Xantrex to see if any current was flowing. You can also back probe the alternator, which I did after the quick check, but be careful. It does not hurt to make sure the alternator is getting a good ground. I did not like the brittle state of my ground wire (grounded to the engine block) so I added a ground buss bar and ran a new ground wire from the back of the block to the buss bar. Now any additional grounds are added to the buss bar and I do not mess with the engine block connection. Also, it goes without saying but I will say it anyway, you should also check the belt tension.
I sent my alternator to an engine shop but it was not salvageable. If your alternator is bad, I think some on this site have purchased an alternator from Parts4engines.com. Just make sure you get the right alternator. It is very expensive to send anything back to the U.K. (like almost as much as the original alternator purchase).
Hope this helps,