A plug gauge for testing the mouth of a container includes a spring carried by a support, a rod having one end coupled to the spring, and a gauge plug coupled to the other end of the rod. A control mechanism supports the rod and is coupled to an electric motor. Motion of the control mechanism by the motor lowers the rod and plug into the container mouth under force of the spring, and thereafter lifts the rod and plug out of the container mouth. In the preferred embodiment of the invention, the spring takes the form of an air spring. A stripper is operatively coupled to the control mechanism in the preferred embodiment of the invention for lowering against the container mouth, as the rod and plug are lowered by the spring and the motor, for holding the container mouth as the rod and plug are lifted from the container mouth. At least one sensor is coupled to the spring in the preferred embodiment of the invention, and is responsive to position of the end of the rod for determining penetration of the plug into the container mouth.