Whenever people are promising some kind of guaranteed return or the investment just doesn't make sense, hold off.

For example lots of ponzis guaranteed a certain percentage of return per week which is simply unsustainable knowing any kind of exponential math. Then you have to think from the perspective of the other party. If they could make money without you (cloud mining and mining contracts), why do they want you involved?

If those any of that doesn't pass the smell test, move along.