2238's Critter::IsRunning() is a conjunction of Critter::IsRuning and Critter:IsMoving(). The latter checks if the critter changed its hex in the last n ms, where n is the time needed for walking one hex for the critter's current crtype, as defined in CritterTypes.cfg.
Critter::IsMoving() also contains a bit of ugly hackery in order to obtain current game time in the DLL, which wasn't directly possible in the time of its writing.