  The precise theoretical characterization of a fractionalized phase in spatial
dimensions higher than one is through the concept of ``topological order''. We
describe a physical effect that is a robust and direct consequence of this
hidden order that should enable a precise experimental characterization of
fractionalized phases. In particular, we propose specific ``smoking-gun''
experiments to unambiguously settle the issue of electron fractionalization in
the underdoped cuprates.
