I would like to spot an enemy jet behind me when on a jet
If an enemy jet is right behind yours, you will know it by its bullets.
The best way to spot a jet its using mouse right click(freelook) and controlling the camera. I usually play inside the cockpit and I never managed to put the mouse center right over it when he is behind me, and it is risk to try it. You can try some maneuvers, to get to his side, under, over or back, it is better than to stay at its aim spamming "Q".
I recommend you to stay way from maneuvers that put your plane perpendicular to the enemy, because in this game it makes you an easy target most of the time, but the immelmann turn is a good one if you are using the fighting jet to land some bombs.