The proof is already there. He and his lawyers knew they had classified documents in New Jersey but kept it secret because they discovered it just before the 2020 election and didn't want it to get out before the election. When that was discovered they should have looked for more classified documents but didn't, with them finally admitting months later that there were more, and then after the DOJ showed up then they found even more.
On Jan. 14, the White House announced that five more documents were found at Biden’s Wilmington residence earlier this week
time.com