An overtaking B777 shows up on the radar altimeter in cruise
Just a quick illustration of how accurate airborne navigation is nowadays, an aircraft on an airway will usually be within 0.05NM laterally of another one on the same airway:
On the left you can see the 777 on the navigation display as a TCAS blip 2000ft below us, then on the PFD the radar altimeter comes alive and indicates roughly 1960ft when the traffic passes below...