You have just detected an airplane. As there shouldn't be other aircraft in the area, you can safely assume it's the enemy aircraft.

Note that usually there are civilian aircraft in the area of operations, so you should clearly identify contacts before firing.

Now it's time to shoot down the intruder before he can collect any intelligence over our territory.

Remember that if you have selected the F-15 and you want to shoot the AIM-7 Sparrow, you will need to keep them pointed at the target with the radar switched on during the entire engagement, otherwise the missile will lose guidance and will self-destruct. If they are ordered to do an automatic attack, they will do this without any additional orders needed from you.