The Stumpy and ACD Compared
The distance from ground to elbow will be longer than the distance from elbow to withers in the Stumpy.
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.
A properly proportioned ACD
A properly-proportioned Stumpy
Other differences exist that the eye might miss at first.
The ACD is longer than he is tall. This is called the length-to-height ratio.
The Stumpy Tail Cattle Dog is a square dog. In other words, he is as tall as he is long.
Whereas the ratio of leg to body depth in the ACD is roughly 50-50...
...the Stumpy has more leg length than body depth.