Charging System Questions

[zzmac]

Hi guys,

Background: Car has only 2000 miles on it, Roush 402R

Car won't start. Gave it a boost and drove car a couple of miles yesterday and guage in car showed fine for half the trip then dropped to maybe 4 volts for last half of trip. When I parked it and shut it off, it then wouldn't turn over. Multimeter guage shows the battery at 12 volts (quite sure it's not the battery).
1. If battery doesn't have enough juice to turn it over, how does it show it at 12 volts on the meter (just curious)?
2. If belts and connections are tight and clean what other than the alternator can it be?
3. Are odds low that it's the alternator with only 2000 miles on the odometer or is that irrelevant?
4. When I get home tonite and check the meter with the car running and it shows less than 13 volts does that mean it's the alternator or can it still be something else?
5. How does a newbie check for things other than the alternator, with a test light?

Just getting back into the mechanic thing after many years. Thanks for your help and suggestions.