> And how difficult it is to identify a civilian plane vs a weapon?

if there is such a way, then your enemy planes can just as easily masquerade as a civilian craft and evade the missile!

The way to make do it work is that a civilian profile would utterly suck as a missle - pbviously 9-11 demonstrated limitations of it.

Air traffic control messages and flight paths could help with that but there are complications - especially when sending out radio communication could light themselves up as a target.

Theoretically fiber optics and dedicated communication towers could separate the functions but there would be considerable logistical implications to that and well - even first world nations with a large military budget don't put that high of a priority on avoiding collateral damage.

> easily masquerade as a civilian craft

How does one do that easily?

Well, if it's only some "signature" that is broadcasted (or not), one could flip a switch and appear as a civilian aircraft.