Alternator? OR: How I wasted $300

[jage]

I replaced my starter batteries (tested bad, like the 3rd set that didn't last 2 years) a few months ago and the van has sat since. Went to get it out and the batteries were too low to start, so I put it on a trickle charger for a few hours. That was enough to get up and running, I drove out to Walmart (30min) checked the voltage and shut it down. The voltage was 12.7 and dropped to 12/11.99 after turning it off. Started fine after 20 minutes in the store, but on the way home I noticed my headlights were yellow and dim, even after cleaning them.

Because of the headlights and the ending voltage I watched on the way home and my voltage hovered around 12.6. By the time I got home it had creeped up to 12.9- but it never seemed like on the way out or back that I got that ~13.7v you get when the alternator kicks in, which is usually a few minutes after starting.

This is totally starter side power, the separator is manually disconnected.

So I'm thinking the alternator is toast. Any reason it wouldn't be that? Any easy way to test?