Some simple things to think about when you think you see a wall cloud.... First is it in the right region of the storm? Are you seeing a low hanging cloud on the forward flank of the storm or the rain free base? Another question to ask is is this feature pointing toward or away from the rain? Shelf clouds generally point out away from the precipitation while wall clouds generally point toward the rain. I will have examples below. Is this feature rising? Slowly rotating? Intense rotation? Or bowing out? Is it possible to have tornadoes that don't have wall clouds associated with them? Absolutely! It is also possible for HP supercells to produce a wall cloud buried within the notch of the storm near the NE quadrant of the supercell. However unless you are tucked your way into the notch you would never see it from a distance away because you will be blocked by wrapping rain curtains.
I am going to post some photos below about what a wall cloud is and what a shelf cloud is. In each of these photos I will identify the feature, where it was, the direction I was facing, and what was going on at the time the picture was taken.
This is another example (although tougher) of wall cloud vs not a wall cloud. We have a supercell in Southern South Dakota on May 24th, 2010. We are looking south south west with a previously tornado warned storm. The heavy rain/hail is to the right and the updraft region was to the left. This storm was screaming north at 60 miles per hour and had strong inflow. At the point in time where this photo was taken a lowering was evident out of the rain free base. I suppose you can call it a wall cloud at this point but 4 minutes later....
A little better defined lowering was now evident. However the storm rapidly diminished in intensity and eventually fell apart a little after this. The key is to look for rotation. There was very broad/unorganized rotation at this point and was not an imminent tornado producer.
Here is an example of something that someone may call a wall cloud because of the lowerings and sinister looking sky but it is nothing more than a harmless shelf cloud. Photo taken June 18th, 2010 near Janesville, WI.
Examples of clear cut wall clouds:
Intense rotation was noted in the three images above. Shot #1 was near Groom, TX 4/22/10, Shot #2 was near Farmington, IL 6/5/10, and Shot #3 was near Kiester, MN 6/17/10.
REMEMBER! NOT EVERY WALL CLOUD IS TORNADIC! THE KEY IS TO BE ABLE TO TELL IF IT IS ROTATING! AGAIN ANY THUNDERSTORM CAN HAVE A WALL CLOUD.