For the first time, a microwave intermodulation technique is used to measure the mechanical resonance directly on packaged and unpackaged RF MEMS capacitive switches with quality factors approaching unity due to air damping. The result is validated by similar measurements in vacuum with much higher quality factors. From the measured resonance frequencies, the residual mechanical stress of the fixed-fixed membrane of the switches is derived and its temperature dependence is analyzed and correlated with that of the pull-in voltage. The present technique offers a convenient means for monitoring the residual stress in RF MEMS devices in both manufacturing and operation. It also allows mechanical and electrical degradation effects to be conveniently separated during life testing of the switches.