You noted in "most of the cases" manuscripts are medieval. We do in fact have some very early manuscripts (I think there are [EDIT: I left out the object here... 5th century biblical manuscripts], and every now and then we get a scrap of something in papyri.

Dating texts often depends on the kind of text you are looking at. Cicero's letters, for instance, come with a date often, or if they don't they reference known contemporary events so we can put them somewhere close to the time they were written. Other texts, for instance Thucydides' history, had to have been written sometime after the war began - and probably had several levels of composition.

Other authors referencing a work can help place it too.

There are things that are tricky. There's an essay supposedly by Quintus Cicero that people have thought was really a rhetorical exercise by some Imperial student. Letters from Cornelia the mother of the Gracchi might be the same kind of thing.

The fact that some sources like these two might be "fakes" in the sense that they might not be actual works by Quintus or Cornelia shouldn't lead you to suspect the whole tradition is manufactured. The amount of stuff that would have to be spontaneously produced in the middle ages and spread geographically far enough to become the basis of European literature is mind boggling - all of literature before the fabrication - AND all literature pre-fabrication would have to have been suppressed. And then there's still the evidence of the papyri that are showing up, unless somehow the literary conspiracy extends to the current day?