Dennis is right, cycling your batteries periodically is the only way to know if they're performing as expected. A voltmeter or voltwatch type of device will give you an instant reading that may catch simple problems, but they can't tell you whether or not the battery can put out enough current to sustain control for a full flight.

Cycling your batteries conditions them and improves their performance while also alerting you when it's time to replace a battery pack so a $12 NiCd battery doesn't cost you a $500 aircraft.