  We develop diagnostics based on gas kinematics to identify the presence of a
bar in an edge-on spiral galaxy and determine its orientation. We use
position-velocity diagrams (PVDs) obtained by projecting edge-on
two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy
potential. We show that when a nuclear spiral is formed, the presence of a gap
in the PVDs, between the signature of the nuclear spiral and that of the outer
parts of the disk, reliably indicates the presence of a bar. This gap is due to
the presence of shocks and inflows in the simulations, leading to a depletion
of the gas in the outer bar region. If no nuclear spiral signature is present
in a PVD, only indirect arguments can be used to argue for the presence of a
bar. The shape of the signature of the nuclear spiral, and to a lesser extent
that of the outer bar region, allows to determine the orientation of the bar
with respect to the line-of-sight. The presence of dust can also help to
discriminate between viewing angles on either side of the bar. Simulations
covering a large fraction of parameter space constrain the bar properties and
mass distribution of observed galaxies. The strongest constraint comes from the
presence or absence of the signature of a nuclear spiral in the PVD.
