"Verifiable proof" means providing evidence or facts that can be checked, confirmed, or proven to be true. It refers to information that can be independently verified or authenticated by reliable sources. Full definition
One of the most effective ways to ensure this is to have all of your material items documented for verifiable proof of their existence and worth. (rentersinsurance.net)
You should check with any legal marketing expert and ask for verifiable proof that they have helped a lawyers and / or law firms with marketing for at least 10,000 hours. (legalmarketingforlawyers.com)