Of course we cannot know anything in the absolute sense when it comes to history, especially history from 2000 years ago. But I do think we can determine what is or what is not likely to be true. I have respect for scholars and historians who study these matters and I don't think they are just wasting their time.I don't know. I've read that the three synoptic gospels were the same book just in 3 different languages. Mathew in Hebrew, Mark in Latin and Luke in Greek. So that the whole empire could read it. We don't have the originals. Short of a archaeological discovery, I don't have a clue how we could know.
I think we can look at the evidence and determine with a reasonable degree of certainty that none of these documents were actually written by someone who was an eyewitness to the events they describe. And eventhough we don't have the original manuscripts we do have good evidence that those original manuscripts were originally written in Greek.