The other night when leaving work I noticed the red battery shaped light on the console was light-up. Also, the battery gauge was lower than normal; I'm thinking that after driving for a few minutes the alternator should charge the battery back to normal, it didn't. After about 5 or 6 miles the needle on the battery charge gauge dropped noticably. By the time I got home the headlamps were noticably dim, and the gauge was buried at 'low'. Alternator, what else?

Truck specs:

1999 F-150
4.2 liter engine-? (large 6 cyl)
6000GVW
2 whl drive

Question #1: How to check with multimeter?

Question #2: How to positively ID, for part replacement?