Ionization probes installed in the head gasket of an engine have been used to infer the shape of the burned volume from measurements of when the flame contacts the gasket. It is demonstrated that the technique can be extended to infer fluid motion by using one of the ionization probes as the ignition site, with the ensuing flame serving as a flow marker. It is shown that swirl motion, and its direction, can be detected, and that flame propagation velocities can be measured. A comparison of estimated swirl velocities with laser Doppler velocimeter measurements show remarkably good agreement. The most valuable feature of the technique is that it can be applied to any production engine without modification.