  In order to understand how molecular clouds form in the Galactic interstellar
medium, we would like to be able to map the structure and kinematics of the gas
flows responsible for forming them. However, doing so is observationally
challenging. CO, the workhorse molecule for studies of molecular clouds, traces
only relatively dense gas and hence only allows us to study those portions of
the clouds that have already assembled. Numerical simulations suggest that the
inflowing gas that forms these clouds is largely composed of CO-dark H2. These
same simulations allow us to explore the usefulness of different tracers of
this CO-dark molecular material, and we use them here to show that the [CII]
fine structure line is potentially a very powerful tracer of this gas and
should be readily detectable using modern instrumentation.
