We write the known invariant definition of the Szekeres-Szafron family of
solutions in an intrinsic, deductive, explicit and algorithmic form. We also
intrinsically characterize the two commonly considered subfamilies, and analyze
other subclasses, also defined by first-order differential conditions.
Furthermore, we present a Rainich-like approach to these metrics.Comment: 12 pages; Accepted in Phys. Rev.