What clues would have indicated KAL 902 was off course?
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).