[Three_Fingers]

'K, if yer gonna use an ammeter:
A.) Ya can get one in an auto parts store or out of an older vehicle that used one. Shunt and all.
B.) The ammeter needs to be wired in SERIES in the hot lead from the alternator-no ground about it.
If you connect your Multimeter like you suggest, the only thing you'll see is smoke as the internal shunt in your meter burns up.
Couple formulas:
Power(Watts(equipment rating))=Volts x Amps
Amps= Watts divided by Volts
Volts= Watts divided by Amps

Best way to check your alt ouput is to do it with a voltmeter.
Set it on the scale that is just a bit higher than the voltage you're measuring.
Take a voltage reading with the engine off/key off.
Should be 11.8-12.5 volts thereabouts.
Now start the engine, voltage should increase-depending on ambient temperature- to about 13.8-15v. You usually only see higher (15v) readings if it's really cold out or the vattery has been severely discharged.
With the engine running-turn on lights, heater or A/C, etc. Stuff that draws heavy current.
If the voltage reading fluctuates lower briefly and then recovers to 13+ volts-everything can safely be assumed to be working OK.
If the voltage reading drops and stays dropped-either you got bad/dirty connections or you need a new voltage regulator.
If you see low voltage at the battery with the engine off and then higher volts with it running-chances are your battery is shot.
Running with a bad battery puts a helluva load on the alternator constantly and that doesn't do it any good.

If you really wanna check your alternator's current output-many auto electrical places and some like Autozone will load-test your battery and alternator for free with a heavy tester.