Actually, what is more important than 'proof' of hardware, which can always be faked, is proof of hashrate. This is extremely easy for the cloud mining operator to show live 24/7, they only need to show blocks being mined as they happened and from this can be extrapolated a rough hashrate.

The only problem with this is establishing that they haven't sold more hashrate than they evidently possess.

Which is why, if you really can't run your own hardware in-house but still want to mine, you're better off avoiding restrictive 'cloud mining' schemes altogether and simply renting actual rigs or straight-up hash-power which you can point wherever you wish.