Shouldn't that be obvious from the power supply? Photos of that unit show an AC power cord connection. Whatever the power supply is producing in DC volts, the batteries should match. If the individual battery voltage is the same as the power supply, they're in parallel. If the battery voltage is 1/3, then they're in series.
That is assuming that the unit is meant to run off of the batteries when there is no AC connection, like rechargeable lead-acid batteries. If the batteries perform some other purpose, you will need to provide a lot more information.