You know how?! Really?! I think that is saying quite a lot.
I'm going to argue that no one really knows what the final clincher on the USA's fall will be. And, given that, no one can really know when it will happen. You have a few choices: 1) get out of the US now while you have a chance, 2) live it up here, 3) prepare for the fall.
I believe It is coming soon. These are MY reasons for believing that (with no intentions of badgering you or any other reader into believing the same, although if you would like to read particular passages for the sake of curiosity, I can tell you specifically what I'm referencing) The book of revelations in the bible discusses the series of "kingdoms" (7 i think)which will control the earth over the course of it's history, with the last one mentioned being the Anglo-American "kingdom". It also includes references to the UN. Several gospels describe in great detail the social conditions which will exist at the very end, and my friend, they mirror the nightly news. These same passages make mention of the numbers and types of natural disasters which will be taking place. Again, my personal beliefs, feel free to disregard.