Alternator dying?

Today when driving to work I noticed my alternator light on at idle but dissappeared when moved. Then when I left work the light was on until I hit 3000rpm, but once got onto the freeway doing 110km/h the light would occasionally flash at me and noticed my dash lights were completely pointless as they weren't bright enough and my head lights weren't as bright. Indicators were barely working, couldn't even here the relay like usual. Once I got home I put the battery on charge and the charger only read 11volts, in the 15 minutes its been charging for its jumped up to 12.9v and put in 900amps.
I do plan to put in the alternator from the 205 block I have laying around, but I don't know how good it is and weather or not it is big enough to handle the electrics in my RS.
I want to know how do you test an alternator, I remember something to do with a volt meter while the car is running.
And what are some other areas in the electrical system to check for.

It sounds like the brushes in the regulator are worn out, hence why when the car idles the light comes on but when revved it goes of (most of the time). The way to test is:
Start the engine.
Using a volt meter check the voltage coming out of the alternator, usually a thick red wire on the back.
At idle the alternator should put out about 14.7 volts or there abouts.
Now turn on the heater fan, headlights (high beam), wipers, radio etc whilst the engine is still running.
Again check the voltage at the alternator. You will need a minimum of 13 volts to ensure that the electrics will operate as well as charge the battery. If you find that the voltage is any lower than this it points to the brushes in the alternator being worn out. These are easy to replace if you know how. If you have a workshop manual it will show you how this is done or alternatively give me a call and iI will talk you thru it. I wouldnt recommend removing the positive battery lead whilst the engine is running as the alternator thinks that the battery is completely flat and over excites the rectifiers inside the alternator. this can result in damage to the internals.
0414 373 388
Cheers Craig

This one of the worst things to do, if the alternator is not faulty it will jack the output voltage up through the roof plus the alternator will attempt to put out as much current as possible. If the positive terminal earths out you WILL **** the alternator.

Any battery place should be able to test it for free. 30 second job