This works for all batteries in portable Mac models.
• Pull down the () Apple menu and choose "About This Mac".
• Click the "System Report…" button.
• Under Hardware, select "Power" and look for "Cycle Count" under the "Battery Information" portion.
The number shown is the cycle count of the current battery. A battery charge cycle is when the battery has been drained to 0% and then refilled to 100% of it's maximum capacity. Knowing the cycle count is helpful if you suspect your battery is having problems retaining a charge. Apple says new notebook batteries are designed to retain 80% of original capacity after 1000 cycles. If your battery is performing at less than expected and is still under warranty, it may be a good idea to schedule an appointment with an Apple Genius.