I cleaned my wife's EightStar last night. Of course, I had to ogle it a bit under the loupe before handing it back.
Look at the EightStar under the loupe. Notice how all eight 'arrows' are visible simultaneously. Notice how all the arrow shafts and all the arrow heads are equal in size and shape. Notice how the arrow heads are properly aligned with the arrow shafts and are straight, not tilted.
Now look at another well cut diamond --you won't be able to find the pattern in a poorly cut one. Are all eight arrow shafts and arrow heads visible simultaneously? Or you see shafts with no heads, tilted heads, misshapen heads, etc. How do their sizes and shapes compare to the EightStar?
This comparison can be made with no special instrumentation --just the loupe. Try it.