A proof is a decision, not a preview
If nobody can say which file was approved, the press sheet becomes the argument.
What approval means
Approving a proof is agreeing to a specific file, at a specific size, with a specific set of open items. A verbal yes covers none of those. When the press sheet arrives with a missing variable line, the only useful question is which proof the client actually saw.
Check the blockers first
Trim size, bleed, spine, copy and colour are separate checks. A proof that is beautiful but scaled to fit a screen has already lost four of them. Read it at 100 per cent on paper, and read the panels nobody looks at: spine, back cover, inside flap.
The check to keep
Put the proof version, the trim size, the approver and the date into the sign-off sheet. List open items separately. Store the signed sheet with the final PDF so the next question has an answer.