Methods for testing the hermeticity of casings for power sources intended to power implantable medical device by sensing the presence of vapors escaping from an electrolyte contained therein are described. More broadly, the present leak detection methods are applicable to any sealed enclosure having a first part sealed to a second part with a liquid contained therein. The liquid need not occupy the entire volume of the enclosure, but must contain at least one component having a vapor pressure at 25° C. of more than about 0.1 mm Hg. This component can assist in the functioning of the device such as an electrolyte, or be added for the sole purpose of leak detection.