Well... to start with, too little is visible. In dry specimens, hairs cause reflections and the otherwise properly visible internal structures of the Clubiona females get obscured. So basically there is little to see externally - many Clubiona epigynes are rather inconspicuous, consisting of the (usually small) atrial opening(s) and the posterior epigynal margin. In terrestris, the middle part of the margin seems convex while it is rather flat and doesn't continue as posteriorly as the lateral "lobes" in phragmitis. Also, in terrestris the internal structures are usually larger and continue a lot more forwards and would be visible there even as a shadow. And don't forget Clubiona phragmitis has very thick chelicerae.