  Assessing the quality of an ensemble of noisy entangled states is a central
task in quantum information processing. Usually this is done by measuring and
hence destroying multiple copies, from which state tomography or fidelity
estimation can be employed to characterize states. Here we propose several
methods to directly distinguish between two different sets of states, e.g. if
their fidelity is above or below a certain threshold value. This turns out to
be significantly more efficient, and importantly keeps the verified states
intact. We make use of auxiliary entanglement or an ensemble of larger size,
where we operate on the whole ensemble, but measure only a small fraction where
information has been concentrated. For certain state families, we demonstrate
that such an approach can even outperform optimal methods that collectively
measure directly a fixed fraction of the ensemble.
