Tips
Some tips to determine rotation in hard to determine situations
1. If you are not close enough to the circulation/meso, distance may cause you to not be able to ascertain rotation. This would be a "suspicious lowering" type of situation. If it is safe to do so, getting a little closer to confirm rotation would be helpful.
2. The suspect cloud's edges will change -- will not be static. A non-rotating cloud formation will have very little changes with the edges -- or perhaps a slow UPWARD motion. Upward however does not mean definitely indicate rotation - but does indicate the presence of an updraft.
3. Look AROUND the suspect cloud for rotating clouds or cloud formations. It should be fairly easy to ascertain rotation as the clouds to the front of the formation will move from left to right and the clouds behind the formation will move right to left in a typical cyclonic tornado situation, and vice-versa for anticyclonic situations.
4. Most importantly, do not make a rash decision -- take a minute or two to study the feature if you are not certain. If you are in doubt, say so.
5. Identify between sustained rotation and short spin-up type rotation. "Eddies" on forward flanks may briefly exhibit rotation but are usually not tornadic threats. In meso situations on the business end of the storm -- If its rotating fast enough to tornado, you won't have much doubt about it.
As was mentioned, you can never tell from a still picture what is and is not a rotating feature, no matter how convincing it would look -- you'd have to study video or "have been there".
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
There's actually a great page out there on the web on tornado lookalikes including smokenadoes, scud features, inflow lookalikes and others. WIsh I had that link available while I am here at work.
Hope this is helpful.