how do you test an alternator to check if it's charging properly?

I just picked up a multi-meter, I figured that is what I would need, but what setting would I even use?

I have an electric fan and if it's hot the thing turns on as soon as you turn the key to "on" and it draws all the current and then it won't crank well. This happened at a gas station today and then of course once it didn't start the first time it was that much harder to start the next crank. When all was said and done I had to get a jumper cable hooked up cause it wouldn't even crank.

I heard a good battery is closer to 13 volts than 12, according to my volt meter mine is 12 at best.