I have just been working on a Macbook Pro (2005) and they asked how they can find out information on how the battery health is. They are finding that when running on battery, the time they get to use the laptop before having to plug it in to a charger is getting less and less.
Below are a couple of quick and easy ways to diagnose the health of your battery. 
These screen shots where taken on my Macbook Pro, from using the methods I mention you can see how bad my battery currently is.
The first way is to look at System Profiler. You can find the from the Apple menu -> About this Mac, then you will be faced with a picture like the one below.
From here, click on the More Info button, it will then load System Profiler. If you look under the Hardware Section on the left hand side of the screen, there is a section called Power.
Then on the right hand side you will see all the details on your battery, as you can see mine has seen better days.
The other method I recommend is that you get this handy little utility installed on your Macbooks, 
it's called Battery Health Monitor which can be found here.
It's a really good little graphical utility that enables you to see all of your battery information.
You can see a screen shot of the application below.
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