98 Caravan ,Dead battery

I went to start my van the other day and the battery is dead. I dont think I left any lights on. I think they turn off anyhow after a period of so many minutes I did recenlty install a CD changer and I wonder if that may be it. It goes off with the key though. I just got the van a week ago. I remember there is a way to test battery drain with a voltmeter by removing the cable and checking volatge between the clamp and post.
I just dont remember which post (positive or negative) and what the voltmeter should read. (You then can remove fuses to see what circuit the problem is in) Can anyone help me ?