I'm using an app called "Battery Indicator".
It's from the App Store that you access with your Captivate. Free of charge it's only 548kb in size and runs in the background with no hit to battery life. It places a small horizontal icon that looks like a battery along the top of the display.
Reading out in percentage remaining, it's quite accurate and a good gauge of how much you have remaining. Just install it, and it sets itself up automatically.
I am really pleased with the excellent battery life I'm getting on my un-rooted Captivate with heavy usage on the net, email etc.
I get better life than I do from my Droid X or my iPhone 4. It took about five charge and discharge cycles for the battery to begin returning good life, as in the beginning it was not that great. I'd say about the third week of use, is when the battery began delivering longer run times.
Cheers...