Rain shafts may look like tornadoes, especially thin ones.  However, to tell a rain shaft apart from a tornado, observe the motion within the feature.  A tornado will be rapidly rotating, and often will have a debris cloud at the surface.  The rapid motion within a tornado will usually cause it to change shape and form rather quickly.  On the other hand, the only motion evident within a rain shaft will be slow downward motion, and the motion of the rain moving with the storm.  Rain shafts may also change form but will usually do so much slower than a tornado.