How can you tell if a battery is about to kick the bucket?

[transit360]

I have two identical buses. One bus has steady voltage almost
always and the other varies from about 14.5 volts to 12.5 volts. I
can't figure out where the voltage draw is going. It almost acts like
some as large as an electric cooling fan is coming on, but these buses
have hydraulic fans. And the other bus never shows syptoms like this.
So I'm wondering...is one of my batteries about to die? I've
heard the lead plates inside can "almost" touch, and cause problems
since they're almost a dead cell, so is there any way I can test these
batteries or do I just have to wait 'til they drop a cell?

Steve
Two '98 Q-buses