There are several apps that are battery meters, most are free, go to app store and do a search for "battery meter" and you will get a list of a few, chose the one you want and download! I have a couple different ones, good luck! There is an app for almost anything or big tv show a available! Check em Out!
If I remember correctly the iPhone 3G did not have that feature. It was introduced in the iPhone 3GS. if your iPhone is jailbroken you can get the "iPhone 3GS Battery" download to display the percent of your battery.