Criteria to help you spot a cloud mining scam/ponzi.

1) No public mining address / no user selectable pool.
A cloud mining company that wont let you direct the hashrate to your pool of choosing and cant prove its actually mining bitcoins itself, is very likely a scam. There is no reason to hide mining address or not sign blocks. None.

2) No endorsement from any asic vendor
Asic vendors will gladly make a simple post to show the company in question is a significant customer of theirs. Its free advertisement for them and it helps their customer grow their business, so there is absolutely no reason they wouldnt. If a (cloud) mining company cant get any asic vendor to post such endorsement, you should assume they dont have any hardware to mine with.

3) No relevant pictures of their hardware and datacenter
There is no reason not to provide such pictures, except of course, if there is nothing to take pictures off. Mind you: pictures can be faked. Picture dont prove current ownership. So like all criteria listed here, by themselves they are by far insufficient proof.

4) Open ended IPO / fractional reverse mining risk
Unless the cloud mining is operated by the asic vendor himself, you can not sell an unlimited amount of hashrate. Hardware takes (usually a long) time to order, arrive and deploy. Any company that doesnt limit sales or make public how much hashrate they sold vs what they have (provably) deployed should be considered  suspicious. 

5) Referral programs and social networking
Referral programs, especially ones that pay almost 10%, are a huge red flag. The mining market is cut throat with razor thin margins. No real company can afford to pay 10% referrals on below market cloudmining prices. Referral programs almost always serve only to feed the ponzi and provide financial incentive to posters to lie about the true nature of the company. Never trust anyone with a referral link in their sig.

6) Anonymous operators
If the operators are hiding behind whoisguard, provide no provable identity and especially when, like in some cloudmining cases, they use demonstrably false ID or company registration information, you have to be nuts to trust them with your money.

7) No exit strategy
If you cant sell your position, you cant get your money out. Thats the ideal case for a ponzi and allows it to run for a  long time.

8 ) Bonus point for "guaranteed profit"
So far, Ive only seen bitcoinmaker.ch do this. If anyone guarantees you a bitcoin denominated profit, and especially a 30+% one, you can be sure its a ponzi, all the other criteria become unimportant. There is no such thing as certain profit when it comes to mining, no one knows how the network will evolve, or what btc exchange rate will do. If anyone could somehow be certain of making a 30% profit, they wouldnt need your money (and they wouldnt give the profit to you).