  Scintillation spectra of some pulsars have suggested the existence of
$\lesssim$ AU scale density structures in the ionized interstellar medium,
whose astrophysical correspondence is still a mystery. The detailed study of
Brisken et al. suggested two possible morphologies for these structures: a
parallel set of filaments or sheets (the `parallel stripes model'), or a
filament broken up into denser knots (the `threaded beads model'). Here we
propose a straightforward test that can distinguish these two morphologies:
whether the apex of the main parabolic arc created by the scattered images
deviates from the origin of the scintillation spectrum or not. In the `parallel
stripes' model, the scattered images move along the stripes as the relative
position of the pulsar moves. As a result, the pulsar is always co-linear with
the scattered images, and thus the apex of the main parabolic arc stays at the
origin of the scintillation spectrum. In the `threaded beads' model, the
scattered images remain at almost fixed positions relative to the density
structures, and thus the pulsar is not co-linear with the scattered images at
most times, leading to an offset between the apex and the origin. Looking for
this possible offset in a large sample of pulsar scintillation spectra, or
monitoring the evolution of parabolic arcs will help pin down the morphology of
these tiny density structures and constrain their astrophysical origin.
