The most obvious differences between the ACD and the Stumpy, other than tails (or the lack thereof), is color. 

The correct blue Stumpy has no tan markings, but the blue ACD does.

Other differences exist that the eye might miss at first.

The Stumpy, at first glance, appears to resemble the Australian Cattle Dog. Closer examination reveals that apart from the absence of tail, there are several major differences between the two breeds. 

The body is square and thus appears leggier. It has length of leg like the Dingo, so as to allow the distance from the elbow to ground to be more than half that from the withers to ground.
 
The ears are moderately small, pricked and almost pointed and set higher on the head than those of the Australian Cattle Dog. 

The head tends to be more 'sculpted'  and less 'heavy' than that of the ACD. 

The Stumpy has a high-set, undocked tail. The true stumpy tail should not be longer than four inches in length, and should not be carried much above the level of the back. 

The Stumpy has no tan on the legs or face. Due to the absence of the black and tan Kelpie in the makeup of the Stumpy, it does not possess the Black and Tan gene -- thus its color is blue, blue speckled or blue mottled. Both the head and body may have black markings, but tan markings are not allowed. Up until the '70s, tan was accepted. (It is said that the presence of tan indicates "back crosses"  to the Australian Cattle Dog. It was known that some breeders did cross their ACDs with Stumpys in the early days; however, that practice was not acceptable then, nor is it acceptable now.) The red speckle must be a good red speckle all over, and darker red markings are allowed on head and body. 

The gait of the two breeds is influenced by the variation in the height-to-length ratios, square (as 10 is to 10) as opposed to longer (as 10 is to 9). The Australian Cattle Dog has more angulation, therefore, it has greater length of stride. The Stumpy has a tendency toward an ambling movement at slow speed.