I mean, pretty much every photo and video I've seen of the guy comes from the guardian - they could easily just be of a Guardian intern. So a photo of him holding a sign is out. A GPG key which is reputedly his is of extremely questionable provenance so a signature from that would be meaningless. He could leak some new top secret data to establish that he has access to it - but we know guardian journalists have had access to such data already and surely they have some stuff on file for future reports.
I mean, pretty much every photo and video I've seen of the guy comes from the guardian - they could easily just be of a Guardian intern. So a photo of him holding a sign is out. A GPG key which is reputedly his is of extremely questionable provenance so a signature from that would be meaningless. He could leak some new top secret data to establish that he has access to it - but we know guardian journalists have had access to such data already and surely they have some stuff on file for future reports.
I'm curious as to what sort of proof you'd like?