A tether subassembly, which may be employed by a tool that deploys an implantable medical device, includes a test segment for verification of adequate fixation of the device at an implant site. When the device is located in proximity to a distal opening of the tube, a tether first length extends through an attachment structure of the device and within an elongate tube of the tool, a tether second length extends alongside the tether first length within the tube, and the test segment is located in proximity to the distal opening. The test segment is configured so that only a tug force, applied to the tether first length, and greater than or equal to a predetermined force, can pull the test segment through an aperture, either of the delivery tool or of the device. The predetermined force corresponds to a minimum adequate fixation force for the device.