  We investigate two-photon entangled states using two important degrees of
freedom of the electromagnetic field, namely orbital angular momentum (OAM) and
spin angular momentum. For photons propagating in the same direction we apply
the idea of $\textit{entanglement duality}$ and develop schemes to do
$\textit{entanglement sorting}$ based either on OAM or polarization. In each
case the entanglement is tested using appropriate witnesses. We finally present
generalizations of these ideas to three- and four-photon entangled states.
