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Our mouse's batteries are in good shape, but we lost our wireless keyboard.
It depends on who made your keyboard and mouse. Apple's wireless mouse and keyboard both report their respective battery levels back to your Mac, for your perusal via System Preferences > Mouse & Keyboard > Bluetooth. If you use non-Apple wireless devices, check for battery levels under the Wireless setting in your device's driver software or System Preferences pane.
Links:
[1] http://www.apple.com/keyboard/