i think a legit cloud mining site will allow you to transfer your mining power to an external pool and mine whatever coin you want
also you will able to see how much hash rate you getting
others are just ponzi sites
This is probably the best way for a cloud mining operator to prove they have actual hashpower. I don't think it would be that difficulty to implement and am not sure why they do not do this.
Around the time that ghash had ~51% of the network hashrate they said they would be implementing this feature in the near future, but it doens't look like anything ever came of this promise.
You would not be able to mine
any coin that you wanted, only coins that have the same mining algo of the contract