Wall clouds are a sign that a storm may produce a tornado, and should be reported to the NWS as soon as safely possible.  When reporting a wall cloud, state if you see rotation (not all wall clouds rotate).  If you do see rotation, specify how fast it is rotating.  The faster the rotation, the more likely the storm is to produce a tornado.  Wall clouds can be tricky to spot, and are easy to confuse with ragged scud clouds and other non-tornadic features.  To tell if what you are seeing is a wall cloud, look for it to be attached to the base of the thunderstorm, subtle upward motion (especially on the side closest to the rain), and possible rotation.  Still not sure?  Call us anyway!  It's always better to be safe than to miss a warning.  Just be sure to describe what you are seeing, and convey that you are not certain what you are looking at.