It's much more likely to be the rectifier than the alternator; alternators seldom give trouble.
The meter should have a 20V DC range, or similar. Mine doesn't actually say DC, it has a logo of a horiz. bar with 3 dots under. You can easily tell the AC ranges from the DC ones though, the DC ranges will be much lower; mine has 200 and 500V AC, and 200m (millivolts), 2000m, 20, 200 and 500V DC. Turn the dial to that (20V), fit the meter leads, and put the probe of the red one on the battery live, and the black on onto a good earth; I use the exhaust mounting lug just above the footrest. You may need to move the earth probe around a little to get the highest reading.
I've just done this on mine and it read 12.18V. Anything over 12V is OK. But well under 13.
Now try with the engine running. Should be around 12.5V to 13V at idle. If it's a little lower (mine was, actually) it's not the end of the world.
Rev the engine and it should go up, but never more than 14.5V.
What you're looking for is figures substantially above or below these. If you get that, suspect the rectifier or its wiring.
Turn the meter dial to "Off" when you've finished.
[Edited on 5-5-2006 by moto748]