[chris_dcosta]

You obviously cannot prove a negative to start with, but in the interests of being non-confrontational, you should just come out and ask what they are looking for in this proof.

I was once asked to prove that future litigation had already been dealt with (my reaction: WTF???) before somebody bought something from me. It turns out they were looking for a specific certification, which was easy enough to obtain.

The point being, they had asked the question in such an odd way, that it just confused rather than communicated.

Just ask them what they expect to receive from you in the way of proof, or that would be acceptable proof, and then go about getting it.