if you still have the factory fitted head unit, if you press and hold the settings button for about 10 seconds you should hear a beep, then by pressing the BC button you can browse the hidden menus on the screen, one of the menus has a battery voltage on it which should read between 11.5 - 12v when the engine is off and go up to around 14.5 with the engine on, if it doesn't go up of fails to go above 13v then the alternator is struggling to charge the battery, simples.