The Lithium Ion battery has a battery gauge (four LEDs) on the same side as the release mechanism. A button above four LEDs can be pressed to show the approximate charge level in the battery. Also, the top right corner of the battery's label indicates that the battery is Lithium Ion. (Li-Ion) is printed on the label.
NiMH
A Nickel Metal Hydride battery has no way of determining the charge level on the battery itself. There is no LED gauge. The label indicates that the battery is NiMH in the top right corner.
Note: If the battery is identified as LiIon, it should be returned to Apple immediately. For more information contact Apple at 800-APL-CARE.