The wiring diagram indicates that the heavy current between the rectifier and the battery do not go thru the headlight nacelle so wiring a ammeter into the circuit might be difficult.
I bought and installed the LED volt meter Scotty's link shows and it works great.
I was able to connect it to my (unused) headlight parking light lead so it's powered any time the headlight or pilot lights are turned on.
I mounted it on the handlebar clamp where it is easily seen.
Here's a link to what is envolved:
http://www.enfieldmotorcycles.com/forum/index.php/topic,12424.msg137999.html#msg137999It's easy to know if the battery and charging circuit are in good shape.
If the first green light is on the system is at or slightly above 12 volts.
If both green lights are on the system is charging the battery with 14 volts.
I guess if I had any complaint about it, it is that the LEDs are very bright. This makes them easy to see in the daytime but their glare at night takes some getting used to.