How to detect network connectivy on an interface?
If they are watching the screen why not go to System Preferences -> Network and watch the connected indicator.  It will show connected when connected to a switch even before DHCP.

This is also where a cable tester comes in handy.  Even the cheesiest of testers will work for this.  Check these out:

https://www.homedepot.com/p/Klein-Tools-LAN-Explorer-Data-Cable-Tester-with-Remote-VDV526-100/309925931

Or a simple tone tester...

https://www.homedepot.com/p/Sperry-Lan-WireTracker-Tone-and-Probe-Wire-Tracer-ET64220/202520187