I bought an Oregon Motorcycle Products rectifier and on his web site he has some great info on testing the Honda twins charging system. Here's some info for the site.
http://www.oregonmotorcycleparts.com/FAQ.html
How do I to test a rectifier?
Start with one lead of the meter (or test light) to the positive lead on the rectifier. Touch the other test lead one at a time to each of the
AC terminals of the rectifier. 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.
4. Charging system diagnostics for older Honda singles and twins (Pre-1978)
First off, read the above articles. These things all need to be dealt with when you have a charging system problem.
There are 3 wires from the stator and depending on the year of your bike they will be different colors. From 1969 on the colors were
pink, yellow and white. The pink wire connects to a full charging coil, the yellow to a 2/3rds coil and the white wire to a 1/3rd coil.
The yellow and white wires connect in the headlight switch when the lights are turned on. What this means to you is that the
resistance from pink to white will be a little less than pink to yellow. You should get around 3/4 to 1 ohm from yellow to pink and
somewhat less from pink to white. None of these 3 wires should have continuity to ground. Check the stator with an ohm meter and
if it passes these test then move on to checking the connections.
The white and yellow wires from the split coils are connected together (in the headlight switch) when the headlight is turned on
directing more power into the charging system. If the switch fails the charging system will not give full output. To make this more
confusing, most pre-1969 Hondas use the same system but with different wire colors from year to year and sometimes model to
model. On these bikes the brown wire usually represents the full charging coil and yellow and pink wires are from the split charging
coil. The best way to be sure is to check the wiring diagram in a repair manual for your bike. The best fix is a new switch but they
are usually no longer available. Some people bypass this and connect the white and yellow wires together at the rectifier giving full
charging all the time. This is only a good idea if you ride with the headlight on all the time.