How can I prove that I did a review for a journal?
Ask the editorial office. You write that you've already asked them, but send them a reminder. Tell explicitly what you're looking for. In my experience for example, a letter saying you performed a review, on official Elsevier letterhead paper &amp; with the desk editor's signature, suffices. This really should be something the journal office can do quickly. If they haven't responded my guess is that they're waiting for the editor-in-chief to respond, which is unnecessary.