A tool of an interventional medical systems system includes a core configured to be temporarily attached to the implantable medical device, as the tool deploys the device to expose a fixation member of the device for engagement with tissue at a target implant site; the core is then employed to verify adequate fixation of the deployed device via a tug test. An operator determines that the device is adequately fixed by the engaged fixation member, if a tug force that is applied to the core modifies the temporary attachment between the core and the device, to allow release of the device from the temporary attachment. A tether, which is fixedly attached to the core, may be employed to create the temporary attachment between the core and the device, or the temporary attachment may be created by a snap fit formed between the core and the attachment structure of the device.