  ALMA continuum observations of thermal emission from the dust component of
protoplanetary disks have revealed an abundance of substructures that may be
interpreted as evidence for embedded planets, but planet-driven spiral arms --
perhaps one of the most compelling lines of evidence -- have proven
comparatively elusive. In this work, we test the capabilities of ALMA to detect
the planet-driven spiral signal in continuum emission. Carrying out
hydrodynamic simulations and radiative transfer calculations, we present
synthetic Band 7 continuum images for a wide range of disk and observing
conditions. We show that thermal mass planets at tens of au typically drive
spirals detectable within a few hours of integration time, and the detectable
planet mass may be as low as $\sim$Neptune mass ($0.3 \, M_{\rm th}$). The
grains probed by ALMA form spirals morphologically identical to the underlying
gas spiral. The temperature of the dust spiral is crucial in determining its
contrast, and spirals are easier to detect in disks with an adiabatic equation
of state and longer cooling times. Resolving the spiral is not necessary for
its detection; with the help of residual maps, the optimal beam size is a few
times the spiral width at a constant noise level. Finally, we show how the
presence of gaps and rings can impair our ability to recognize co-located
spirals. Our work demonstrates the planet-finding potential of the current
design specification of ALMA, and suggests that observing capability is not the
bottleneck in searching for spirals induced by thermal mass planets.
