The study of radiative $\phi$ decays into scalar mesons, with subsequent
decay into $\pi\pi\gamma$, constitutes an important topic at the
electron-positron collider DAPHNE. The interference of the respective
amplitude with the one for $\pi^+\pi^-\gamma$ production, where
the photon originates from initial state radiation, will allow for unambiguous
tests of models for the $\phi\to\gamma f_0(\to \pi\pi)$ amplitude.
The forward-backward asymmetry of charged pions, which is a clear signal of
such an interference, amounts up to 30 \% and is at the same time
quite sensitive to the details of the various amplitudes.
The results for several characteristic cases are presented and their
implementation into the Monte Carlo generator PHOKHARA is described.