This fern and the closely related and more frequent species Polypodium vulgare can hardly be distinguished with the naked eye. but there are clear differences in microscopic attributes as e.g. the size of the spores. The fern from Hemsbach (Bergstraße, Baden-Württemberg, South-West Germany) has been checked microscopically, and most probably also the fern from the Botanical Garden of the University of Heidelberg, where each a group of plants with label 'Polypodium vulgare' and 'Polypodium interjectum' is available.
Many but not all populations of Polypodium interjectum have some attributes, which are hints for this species: The fronds of P. interjectum are longer in relation to the width, the pinnation doesn't reach the middle rip, and P. interjectum has bigger Sori whose location is visible on the upper side as nubs, and and which are oval rather than round in shape.
The Central European species Polypodium vulgare generates the sori usually in summer, while Polypodium interjectum which is coming from the Mediterranean generates the sori rather in winter.