The answer: They can't.
Are you 100% sure? If you are then I guess you have nothing to worry about. I'm not 100% sure that they can't or that they won't be able to in the very near future.
The only way in which they could is if they were pulling off the equivalent of a 51% attack on the TOR network. Which they basically don't have. If you don't trust me, trust the NSA themselves:
http://www.theguardian.com/world/interactive/2013/oct/04/tor-stinks-nsa-presentation-document