  The quantum marginal problem asks whether a set of given density matrices are
consistent, i.e., whether they can be the reduced density matrices of a global
quantum state. Not many non-trivial analytic necessary (or sufficient)
conditions are known for the problem in general. We propose a method to detect
consistency of overlapping quantum marginals by considering the separability of
some derived states. Our method works well for the $k$-symmetric extension
problem in general, and for the general overlapping marginal problems in some
cases. Our work is, in some sense, the converse to the well-known $k$-symmetric
extension criterion for separability.
