Any cloud mining can prove it by showing their mining address(es) as well as publishing sold hash power in real time. Showing picture to owning real mining firm DOES NOT prove anything, because if you have 1 Phs and selling 10 Phs and thereby paying the people holding 9 Phs from other people's investment, you are operating a Ponzi in reality.
Good point to add to this
No proof of mining = Proof of no mining!
The only one i've ever considered is hashnest because obvious reasons we already repeated thousands of times. I wonder, tho, if their API can be used to show their total power/sold power Warren is referring to.
Anyone tried?