One of the things that suggests a tornado may be present in
a thunderstorm is a "hook" of high reflectivity located
away from the main body of the storm, often south-west of the storm.
As seen below, the Stoughton tornado displayed such a hook.
(National Weather Service - Milwaukee/Sullivan Weather Forecast Office)
Several other storms in the area on August 18, 2005 also displayed similar hooks and spawned smaller tornados,
as indicated below.
(National Weather Service - Milwaukee/Sullivan Weather Forecast Office)
Relative velocity radar images make it easy to spot rotation.
Where colors are bright, the wind is blowing fast.
If there is a lot of one bright color (red or green) in an area, that could
indicate damaging "straight line" winds.
If, however, two bright colors are right next to each other (red and green),
that indicates that the winds are blowing fast in multiple directions.
The most likely explanation of this behavior involves a tornado.
The bright red and green image below clearly displays the Stoughton tornado
kicking off fast winds in all directions.
(National Weather Service - Milwaukee/Sullivan Weather Forecast Office)
That was easy, but the next image suggests why your local news affiliate doesn't
usually display the "relative velocity" radar image during the newscast.
To the trained eye (to the person who circled them anyway), three
suspicious bright green and red spots stand out, indicating three tornados.
To my untrained eye, however, there seem to be many more likely red/green candidates.
(This is why it's good that I work with computers, not weather forecasts!)
(National Weather Service - Milwaukee/Sullivan Weather Forecast Office)