Discovery.
The court can subpoena Assange for the purposes of his case and then enter the WL documents into the justice system, which can be used in later cases when someone is giving testimony.
"I don't recall…"
Gets met with "does this document here refresh your memory?"
When someone tries to question the legitimacy of the WL documents, that is when subpoenas can go out to intelligence agencies to verify. OR already existing documents (that have been declassified by executive order) can be brought into play to validate what is in WL documents.