As far as I know, none. It'd be rather impossible to know exactly what's stored on someone else's blockchain. Also, it'd be rather difficult to even connect to nearly every user on the network to see who's broadcasting what. For example, my wallet client only connects to two trusted* nodes. For any transaction that I do, it's broadcast through those two nodes.

Then again, I could be wrong about everything I just said.

* I trust these nodes personally, they aren't globally trusted