How can I find out the voltage of the battery on a 2009 Honda civic coupe

A multi-meter or even a simple voltage meter. If you want strictly battery voltage, check with car off. You just put the red lead on the red +Pos terminal and the Black lead on the Black --Neg battery terminal. Be sure your meter is set for voltage and in the proper voltage range, it may have a high voltage setting for checking household voltage in the 120V range But most auto parts houses will check it for you free

But remember, if you are having battery trouble it may show 12-13.5 Volts, but that does not mean it is pushing out enough amperage, a different thing. It needs to be load-tested.

Go to your local auto store big box franchise and check it! They will do it for free! What this does is find out how much cranking power is being used to start the car and to determine if the charging system of the car is adequatelt RECHARGING THE BATTERY ONCE STARTED, i use caps to emphasize they are doing two separate checks. 1 on the battery 2nd on the system alternator/voltage regulator ! One side note if you are going to DIY make sure multimeter is on DC Direct Current and not AC alternating current which some of the higher end units have! Make sure that pointer is at least on 20 volts or higher to get a correct reading NOT. Any reading point below 14 !hope this helps your understanding