When astronomers study the dark matter halos of spiral galaxies, they normally assume that the disk mass-to-light ratio is constant. We describe a method of analyzing the kinematics of planetary nebulae (PNe) in nearby face-on spiral galaxies to test this assumption. Since the restoring force for stellar motions perpendicular to the galactic disk is proportional to the disk mass surface density, measurements of the vertical velocity dispersion can be used to produce an independent measure of the total amount of matter in the disk. Our steps are: (1) to identify a population of PNe by imaging the host spiral in several filters, and (2) to isolate the vertical velocity dispersion from spectroscopic observations of the PNe. Our first results for the PNe of M33 indicate that the mass-to-light ratio of the galaxy's disk actually increases by more than a factor of 5 over the inner 6 disk scale lengths. We have begun similar studies of the PNe in five more face-on galaxies: M83, M101, M94, NGC 6946, and M74. These data will also produce additional science such as galaxy distances and constraints on the disk transparency.