  If the dark matter halos of galaxies contain large numbers of subhalos as
predicted by the $\Lambda$CDM model, these subhalos are expected to appear in
strong galaxy--galaxy lens systems as small--scale perturbations in individual
images. We simulate observations of multiply--lensed sub--mm galaxies at
$z\sim2$ as a probe of the dark matter halo of a lens galaxy at $z\sim0.5$. We
present detection limits for dark substructures based on a visibility plane
analysis of simulated Atacama Large Millimeter/submillimeter Array (ALMA) data
in bands 7, 8 and 9. We explore two effects: local surface brightness anomalies
on angular scales similar to the Einstein radius and the astrometric shift of
macroimages. This improves the sensitivity of our lens modeling to the mass of
the lens perturber. We investigate the sensitivity of the detection of
low--mass subhalos to the projected position of the subhalo on the image plane
as well as the source structure and inner density profile of the lens. We
demonstrate that, using the most extended ALMA configuration, pseudo-Jaffe
subhalos can be detected with 99% confidence down to $M = 10^7 M_\odot$. We
show how the detection threshold for the three ALMA bands depends on the
projected position of the subhalo with respect to the lensed images and
conclude that, despite the highest nominal angular resolution, band 9 provides
the poorest sensitivity due to observational noise. All simulations use the
\emph{ALMA Full ops most extended} ALMA configuration setup in CASA.
