How do pilots of big jets determine if they are clear of the runway after landing?
Using TLAR (That Looks About Right).  Pilot turns off a runway onto a taxiway spur and sees the runway safety area lines and just estimates a point beyond roughly equal to or greater than the length of the fuselage and places the cockpit over that point.

Other methods include geo-referenced airport diagrams on a moving map display showing the position of the aircraft.