First off, read this
thread and it will explain why you don't need to worry about background apps. Android will let you see what apps are using up your battery (you'll probably find the display is the biggest power hog). Apps that are in the background are typically idle, not consuming resources or power. If the OS needs the memory they occupy, it sends them a signal, they save their state and exit, freeing the memory, otherwise they wait to be brought back to the foreground. There are some tasks in the background that are working, but even those spend most of their time waiting on a timer or for something to come in over the network and are idle the rest. All worrying about background apps will do is drive you crazy and have little to no effect on your phone's performance.