  Recently we derived generalized sheet transition conditions (GSTCs) for
electromagnetic fields at the surface of a metascreen (a metasurface with a
``fishnet'' structure, i.~e., a periodic array of arbitrary spaced apertures in
a relatively impenetrable surface). The parameters in these GSTCs are
interpreted as effective surface susceptibilities and surface porosities, which
themselves are related to the geometry of the apertures that constitute the
metascreen. In this paper, we use these GSTCs to derive the plane-wave
reflection ($R$) and transmission ($T$) coefficients of a symmetric metascreen,
expressed in terms of these surface parameters. From these equations, we
develop a retrieval approach for determining the uniquely defined effective
surface susceptibilities and surface porosities that characterize the
metascreen from measured or simulated data for the $R$ and $T$ coefficients. We
present the retrieved surface parameters for metascreens composed of five
different types of apertures (circular holes, square holes, crosses, slots, and
a square aperture filled with a high-contrast dielectric). The last example
exhibits interesting resonances at frequencies where no resonances exist when
the aperture is not filled, which opens up the possibility of designing
metasurfaces with unique filtering properties. The retrieved surface parameters
are validated by comparing them to other approaches.
