Sure, you can check how well a battery bounces to test how full it is, but when it comes to lithium or "button" cells, there's a more gentle approach — a pair of DIY tweezers that you can keep with you in your tool kit.
A video with instructions on how to solder and build the tweezers is available over at MAKE, with a step-by-step guide for those after a written approach. The tweezers feature a pair of LEDs to show the battery voltage (and to make polarity irrelevant).
The electronic part of the project isn't too complicated, in fact, you'll spend more time constructing the tweezers. The quality of the build is up to you — you can just get the basic testing setup going, or nice it up by applying heat shrink tubing to each arm.
Of course, you can go simpler still and just carry an LED with you, but that's nowhere near as fun.
Battery Testing Tweezers [MAKE]