To be a bit nicer, what you're engaging in is deconstructionism. The problem with deconstructionism is it can go on ad infinitum and ad nauseum. No matter what the opposition responds with, the deconstructionist can always resort to demonstrating that that answer can't be verifiably proven or known. It's like saying there is no such thing as daytime because we can't prove at what exact point nighttime becomes daytime. But the flaw with deconstructionism is parameters aren't the deciding factor. Common sense is. For example, not knowing at what exact time night became day doesn't stop me from walking outside at high noon and saying, "Whoa! It's bright out! Must be daytime." Can't prove it....
So, how do you determine a prophecy is fulfilled...well, it's fulfilled. Otherwise we start to play the pedantic deconstructionist game where nothing can ever be reasonably known.