You're right, there isn't any proof, unfortunately. I attempted to prove it through Shade on N1, but for reasons unknown to me he doesn't seem to know that I targeted him. N2 I showed up just on time to send my action in only to find out that that deadline changed, so unfortunately I did nothing.
I also can't do anything right now, not having a day action. However, I don't think I'm in any position where I need to prove anything.
Did you start with the Mask in your possession?