The fact that the destruction of Jerusalem in 70 AD wasn’t even hinted at in the books of the New Testament is the most powerful evidence that they were written before that time. This was a pivotal event in Jewish history, and it would have been hard for Jewish writers writing about time to ignore it.
As an analogy, think of someone writing about and events in 20th century Germany, and making no reference to WWII. Even if you are writing about 1910 or 1990 Germany, at some point a reference to WWII would probably work its way in.
As I suggested earlier, the reason this common sense conclusion is not generally accepted is that the liberals who wanted to deconstruct the Bible was able, by the lack of historical evidence, to postulate a second Century writing of the Gospels. Somewhat mischievously, Bishop Robinson even says that a case can be made that John’s was the earliest Gospel, although not in its final state. Other scholars have noticed that it does seem to present something closer to an actual chronology of Our Lord’s career and to include details that demand an Eye-witness. Such as “John’s” knowing how to finagle his and Peter’s way into the house of the high priest after Jesus was seized.