  For pseudoscalar meson electroproduction off the nucleon in parallel
kinematics a relation between three polarization observables is derived. It is
shown that, without Rosenbluth separation, a measurement of the longitudinal
strength can be achieved through three different ways. They are discussed with
preliminary MAMI data for the p(\vec e, e'\vec p)\pi^0 reaction in the energy
range of the Delta(1232) resonance.
