Active dust devils (atmospheric vortices with entrained dust) seen on Mars leave surface tracks inferred to result from the injection of loose particles into the atmosphere, typically exposing a darker (or, in some cases brighter) substrate. Hundreds of similar appearing surface tracks are common in many areas, but in the absence of imaging the active dust devils responsible for their formation, it has not been possible to determine the direction of forward motion of the vortex. Laboratory experiments simulating dust devils show that overlapping scallops in some tracks enable the direction of forward motion to be determined.