I can usually tell (I say "usually" but I'd like to think "always") if a plane is falling backwards without it's tail or diving just as easily as I can make out a missing wing. And I've seen guys call out a kill when all they've done is knock off someone's flap or something (and I've had that "dead" plane come around and get me when I broke off). So yeah, there's times when you gotta use your judgement and observation skills. Sometimes players DO make mistakes and it's cool when they own up to it. Of course, after three or four times in a row it gets kinda suspicious, heh. But, like I said, wingless or tail-less planes are pretty easy for me to ID.
p.s. A burning plane doesn't neccesarily mean it's no longer a threat, either.