Sorry Hope - ancient documents aren't stamped 'Fiction' and 'Non fiction' or 'Allegorical' or 'Historical' and actually there is massive blurring between the two in each case.
Yet there are plenty of linguistic means that we use to show these differences; are you saying that ancient writers didn't know about them, or have their own?
But as you have pointed out below these stories weren't necessarily rigidly one or the other.
And the notion that an allegory must be embedded in a purely fictional story is also not the case. So there are plenty of allegorical tales, which include some or many fictional elements that also include factual and historical elements too.
And nothing I have said indicates anything other than just that. For instance, the book of Jonah refers to Nineveh - a historical place.
Indeed, just as the resurrection story refers to a historical place and perhaps also a historical person, just as Jonah may have been a real person. But that doesn't mean that that in either case the story is historical in full nor partly allegorical which I think is the case in both instances.
So the point is that you cannot assume that if something is not deliberately allegorical that it is necessarily true or historical, nor that the current allegorical 'orthodoxy' from a story is the same as its original meaning.
But if one can trace the meaning of an allegory from close to its origin, then it may well be that you can. After all, Jonah is within that section of the Tanakh that the Jews regard as prophetic - so not necessarily historical or factual.
But whether a story is thought to be allegorical rather than historical (or vice versa) doesn't mean it is, nor that it was intended to be so by the author. Even if the suggestion of allegory appeared very early (and in the case of the bible we really have no idea what the contemporary readers, and the author actually thought or intended.
As an example it is pretty well an 'accepted' fact that the Narnia stories represent biblical allegory. And this has been a broadly accepted view for decades, so arose within a few years after they were written. Yet C S Lewis always denied this. Now we actually have the recorded view of the author in this case, unlike for the bible yet within no time an accepted view of allegory has arisen that the author denies he intended.
