  Quantum communication channels and quantum memories are the fundamental
building blocks of large-scale quantum communication networks. Estimating their
capacity to transmit and store quantum information is crucial in order to
assess the performance of quantum communication systems, and to detect useful
communication paths among the nodes of future quantum networks. However, the
estimation of quantum capacities is a challenging task for continuous variable
systems, such as the radiation field, for which a complete characterization via
quantum tomography is practically unfeasible. Here we introduce a method for
detecting the quantum capacity of continuous variable communication channels
and memories without performing a full process tomography. Our method works in
the general scenario where the devices are used a finite number of times, can
exhibit correlations across multiple uses, and can change dynamically under the
control of a malicious adversary. The method is experimentally friendly and can
be implemented using only finitely-squeezed states and homodyne measurements.
