Actually, there IS a way....
Figure in the location of the plane during disco. If he is within friendly territory, and has not left friendly territory, it was a disco. If not, this is where the complicated stuff kicks in.
First, check ammo. Has the poor bastard shot more than 75 rounds? This is to quickly check if he has been in combat, but still allows for the erroneous trigger pull. If so, then....
Keep track of the location of enemy planes, and their distance. If he has encountered zero enemy planes, he's clean. If not....
Were they within 5K? This allows for encounters unarmed encounters just at/outside con range. If so.....
Has the fighter lost/gained significant speed/altitude during the time the enemy was near? Were there shots fired? Checking for combat conditions here. If so....
NO PERKIES FOR YOU, YOU LOSE