Some batteries can be tested just by dropping them, but coin cells can be much trickier to test. This DIY pair of battery-testing tweezers can test the polarity and charge of coin cell batteries just by picking them up.
Trying to pinch a flat coin cell battery between the probes of a voltmeter can be a major pain, but this build from Sean Michael Ragan at Make can test them much faster. All you have to do is pinch the batteries with the ends of the tweezers and a light will let you know if the battery has any juice left, as well as telling you which side is the positive side of the cell. The entire build process shouldn't take you more than an afternoon, and once they're ready, they will make a great addition to your tool box. The video above shows you all the tools and materials you'll need to make your own, but you can read more at the link below.
Battery Testing Tweezers [Make]