You could buy a digital multitester (you can find them in many stores, even Sears for less han $ 30). Whit this tool you will be able to check not only the bateries but also many parameters (line to neutral voltage, neutral to ground voltage, etc.).
If you want to check the bateries you need to put the multimeter in DC Volts and expect to read between 10.5 and 14.8 volts per battery (for 12 Volts batteries). Even you could keep the multitester connected and make a dynamic test to the battery bank (for charge and/or discharge cycles).
Jose P. Mir
jpm@jpmir.net
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Jose P. Mir
jpm@jpmir.net