Here is one of the primary reasons that I love being part of a Macintosh User Group. I am always learning cool things. At a meeting tonight one of the participants walked in and stated that she needed to plug in her laptop because she thought her battery was dying. She asked if anyone knew how to test it.
Within three minutes we all had a working solution that one member of the group knew about and shared with the rest of us. The solution is a little application called CoconutBattery. The most current version is 2.6.3. The app is free although donations are accepted. It works with both Intel and non-Intel based Macs.
You can get information about the application and download it here. Like any new application you should move it to your Applications folder for storage. Then just click on the application icon and the battery monitoring window will open.
As you can tell from looking at the information about my machine my battery is starting to wear out, it no longer works at full capacity and does not hold a charge very long. The information also tells me the age of my Mac.
I well remember being at MacWorld back in 2003 when the battery of my Pismo died on me. Buying a new battery in that setting was time consuming and expensive. Had I been more knowledgeable and had something like this, I could have taken care of the problem ahead of time.
By-the-way, it is good practice to occasionally let your battery run all the way down and then recharge back to full capacity.