I've had occasional problems with a dead battery. Sometimes the battery has died after leaving the headlights on for no more than five minutes. Yet the battery itself is only two years old. I replaced the alternator with a rebuilt one last spring (after the old one's pulley experienced meltdown in the middle of a cross country jaunt). There's no telling the quality of this alternator--it'd been sitting on the guy's shelf for years (I swear it was the only compatible alternator in all of Rapid City, SD). We also had to swap the external voltage regulator for a new internal one (my 82 240 K-jet always had an internal one).

So something's not supplying or holding the juice. How can I tell if it's the alternator or the battery? I'm measuring exactly 12v across the battery with everything off. With the motor on, it's a bit over 13v. Is there a way I can check the alternator more specifically?