[Mgs] batteries and starting

[oliver]

it seems that after pulling the engine/tranny and replacing the clutch, i have
new woes to deal with.  the car was out of commission for 2 or 3 months
(various things kept getting in the way), and now i seem to have lost one or
both of the batteries.  yes, i'm still running two 6 volters, and i know i
should replace them with a single 12, but i figured while they were working i
would leave them.
 
after charging them overnight, in series, i still didn't have enough juice.  i
added a 12 volt with jumpers and the car started right up.  so clearly i've
identified the problem. 
 
i am charging them again tonight just to be sure.
 
can someone tell me how to identify which battery is bad or if they have both
finally quit?    using my multimeter across the pair (both 6 volters) i get 12
volts.  i suppose i could pull them and trundle off to sears to have them
checked. 
 
they were in the car when I purchased it in December of 2004, so they are at
least 4 1/2 years old. is it time???
 
thanks!