Yes, there are ways to confirm the age of an old text without having the original text itself.
I’ve seen people basically claim that it’s impossible to know when, say, certain books of the Bible first emerged because we don’t have the original texts. They often use this to argue for a later date in their composition, often to make the argument that their own favored form of Christianity is the older one.
However, there are ways to know that a text goes back pretty far even if you don’t have the original. I’m going to list a few here:
Quotations and references: If a text is quoted or mentioned in documents you actually can date, then you can be pretty sure that the text existed (at least in some form) prior to these documents.
Language: Language is always changing, and it’s often possible to date the contexts of a text based on the language its author used. If the language in a text is consistent with the time and place it was claimed to have been written in, then that’s probably when and where it was written.
Historical accuracy: If an ancient text accurately describes historical events in detail, then there’s a good chance it was written around the same time as these events; or at least not too long afterward.
Of course there are exceptions. Just because horror writers quoted the Necronomicon, doesn’t mean the Necronomicon was a real grimoire that existed prior to the 20th century. But in this case, the fact that nobody mentions or quotes the Necronomicon before the 20th century (even if just to condemn it), plus the fact that there are no copies of the book that date before the 20th century, nor any old books that contain even parts of the Necronomicon, tell us it’s a 20th century invention.
Someone who is well-studied in an ancient language might be able to fake an ancient text, but once again we can look at what other evidence is or isn’t there. Does anyone throughout history actually quote or mention the book, even if just to argue against it or condemn its ideas? (Remember, Christians spilled a lot of ink to argue against their opponents - just take Irenaeus for one example.)
Someone with access to accurate historical documents might be able to write a passable historical fiction. However, if the book proclaims extraordinary and sensational events that somehow nobody else who lived in the same time and place saw fit to record, that somehow nobody throughout history found interesting enough to cite or mention, then that’s a pretty good indication that the book is a recent fabrication.
But yeah, even without the existence of an original text, there are ways we can be reasonably certain that a book dates back to a certain time period.