How do I prove that I have a license to use content?

I see in Anki’s Terms and Conditions that, when an Anki user and deck-creator wants to upload a shared deck, he or she needs to “confirm that the content is entirely their own work, or that they have obtained a license from the intellectual property holder.”

Is there a consent form that I can send to the IP holder to secure his permission? If I’ve already obtained written permission otherwise, can I simply affirm this by checking a box?

In other words, is documentation of permission required, and, if so, what kind of documentation?

Thank you very much.