That has been the million dollar question ever since the FDCPA was implemented.
The FDCPA does not provide any definition of what is or is not adequate verifcation/validation of a debt.
Whenever a statute is open to interpretation, then the courts establish their interpretation. Case law is only binding within a given jurisdiction, and then only when the matter has been determined at the appellate level.
With many appellate jurisdictions and no supreme court decsions setting a uniform standard, there are different case law interpretations amongst thte federal appellante circuits.
Most jurisdictions have a standard that the debt collector is only required to investigate and convey their verification as reasonably supported by that investigtion.
They are not required to provide supporting documentation with their verifcation, or to "prove" the legitimacy of the debt unless the matter gets to court.
Only a judge can then determine on a case by case basis if their verificaition is supported by facts.. Thus, it is usually necessary to get the issue of adequacy of any verification before a judge.
