proof positive
Proof that definitively confirms something; undeniable proof (of something). This document—signed by the defendant—is proof positive that he authorized the transfer of funds. If you're looking for proof positive, you're not going to find it. Some things just don't have a definite answer.
proof ˈpositive
definite or convincing proof: It’s proof positive of her belief in the company that she’s investing her own money in it.