[psychoslave]

You can rather easily infer a earliest possible date based on multiple ways, and of course intersect them to make a more solid hypothesis.

One thing is that human being generally very bad at predicting specific future event, if a text depict such an event, chances are good that the document was written after this event.

There are also things like which script/police was used, as well as other philologists cues.

The material used to make the document can also give you a rough idea of when it was created, even without isotopic analysis: people didn’t have easy access to the same technologies and primary material over time and region.

Of course, counterfeit of something that seems older than it really is always a possibility. Time travel to the past is far less probable though. :)