And if the pilot is simulating instrument conditions, there is a rule: FAR 91.109 "No person may operate a civil aircraft in simulated instrument flight unless - The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown".
If not simulating IMC, it's the pilot's responsibility to see and avoid per 91.113(b)
They do not fly simulated IMC ("under the hood") in F-16s. By the time the pilot gets anywhere near a F-16, they have already trained for IMC in multiple trainer aircraft.
FAR 91 doesn't apply to military aircraft, although military rules require they maintain a visual scan outside the aircraft.
And if the pilot is simulating instrument conditions, there is a rule: FAR 91.109 "No person may operate a civil aircraft in simulated instrument flight unless - The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown".
If not simulating IMC, it's the pilot's responsibility to see and avoid per 91.113(b)