I second @Connor W's suggestion of using Activity Monitor (or "top" from the terminal) to see what's using the most CPU. Unfortunately, that might not show what's using the most GPU, and the GPU has plenty of potential to burn power.
Here are some additional battery-saving tips:
Make sure you have a Flash blocker installed. I like ClickToFlash. It's Safari-only, but there are similar tools for other browsers as well.
Flash is the most notorious CPU/battery hog, and is often running just for ads. I seem to recall a study that estimated that on a Mac with default out-of-the-box settings, under typical web usage, over 10% of your battery life goes to Flash ads alone.
Most Flash blockers also allow you to load native H.264 video from popular video-sharing websites like YouTube and Vimeo. Mac OS X's native video-playback code is much more efficient than playing via Flash.
Avoid using Flash for long-running services like Internet audio streaming (Pandora, Grooveshark, playing podcasts directly from a web page, etc.). Download a native Mac OS X client, or opt for their "HTML5" version, or opt for a standard format audio stream that a native Mac app such as iTunes can play.
Don't keep your backlight on max if you don't need it. Don't power things via USB if you're not on the power adaptor (don't leave iPhones and iPads tethered, eject your flash drives and 3G cards). Turn off Bluetooth and AirPort when not in use.