I believe it is simple, their offer is too good to be true.  If we keep our greed at bay, we can easily see their scammy plot because almost all their offer are those that will trigger our greed and make our brain think irrational.  Aside from that, we also need to educate ourselves and do research in every offer of investment we meet.  Patience and thorough research will always save us from the claws of these scam people.