Go here,
http://www.oregonmotorcycleparts.com/FAQ.html
and read #3 for the correct way to test a rectifier.
3. How do I to test a rectifier?
what setting on my multimeter do I use? Do I do this with engine running?Start with one lead of the meter (or test light) to the positive lead on the rectifier
The red wire coming from the rectifier?. Touch the other test lead one at a time to each of the
AC terminals of the rectifier
not sure what these are?. At this point you will either have continuity or not but it should be the same with all the AC terminals on
the rectifier.
Swap the test leads (still working with the positive terminal of the rectifier) and repeat the test. This test should have the opposite
result as the previous test. Again the result should be the same for each of the AC leads.
Move on to the negative lead off the rectifier and repeat the 2 previous tests. This is test is easier with a test light than with an
electrical meter. The point is to check that power flows one way but not the other and the exact numbers isn’t as important. If your
meter has a buzzer for continuity, this works very well too.
This will catch a bad rectifier 95% of the time. The rest of the time they only fail under load and will usually get pretty hot.