What clues would have indicated KAL 902 was off course? Korean Airlines Flight 902 was shot down by a Soviet fighter in April 1978 after the pilot turned through 150 degrees and entered Russian airspace. The New York Times reported it happened around 8.30 pm Paris time, so this happened during sunset. But if he had kept to the correct course, he should have been flying over the North pole and into the middle of the day.

Given that magnetic compasses are not helpful at this latitude, the aircraft did not have an inertial navigation system, and the Earth's surface (if visible) is presumably similar in both directions, what other clues could the pilot have to his heading?

In other words, for flying at high latitudes in the late 70's, what checks were available to the accuracy of navigation?
According to the German Wikipedia the 707 has been equipped with LORAN-C and VOR. It most likely also had (at least one) 
Gyroscopic Heading Indicator.

Wikipedia does not tell something about this concrete plane but these devices where available since the 1950s and there is no reason why this particular plane should not have them.

So from a technical point of view the crew should have been able to detect the deviation from the planned path, given that they had some training in using all of that equipment, and that not all of them broke altogether during the flight (before the rocket hit).