Prove it. The DOJ/FBI gave him several weeks for his lawyers/staff to go through anything they wanted and that means they had lots of time to remove anything really incriminating.
DJT allowed the FBI to inspect the documents he had, gave them full cooperation, and gave them permission to take anything they wanted and offered to produce anything they asked for. He had already returned several boxes requested by NARA. They inspected the documents, didn't ask for other documents, didn't take any documents with them, and only asked that an extra lock be put on the storage room door which was done.
Biden's lawyers and staff are allowed to be present when DOJ officials search his home. Trump's lawyers and staff were banished from the premises and not allowed to observe anything.