Hi,
This will would be the easiest to measure your banks voltages, using some simple maths to get the top bank's voltage level.
It will identify which bank is lower, but not if it is only one battery in the bank.
Sealed batteries unfortunately do not make individual battery condition easy to establish.
Wet cells of course, all you need is a hydrometer.
You need to find the suspect batteries not try and discharge or charge a complete bank.
If you have a DC clamp meter you can check your link currents, with the banks completely unloaded and with no charging.
Checking the current in each link will tell you where your suspect batteries maybe, the current will be flowing into them and dragging the good ones down.
However if you have a battery with high series resistance, then an individual load test is probably another alternative.
Tom..... 