Hi Bob -
Yes, casting loop shape tells the tale, as does the d-loop. But when stepping back to look at the big picture Spey casting is like a house of cards - one card falls and the rest tumble. I'm not convinced there would be a single diagnostic point of reference as such, rather signs all are valuable - whether heavy anchors, bloody L's, lack of consistencies or Frenchman sipping soup
Let me clarify:
the prescription for a good D-loop may often be the sweep technique...
which depends entirely on the lift technique...
yet correction of a forward cast can be made by correcting that d-loop about half the time...
thus the lift is linked to the forward cast via the sweep and d-loop.
I'd say you need to understand the "linkage"
to properly diagnose faults; and this on a cast-by-cast basis.
A clean Spey cast should result in a good presentation of the fly to the fish, however it should also be efficient, consistent, and require only the minimum amount of effort for a particular cast.
There is a certain sense you get as an instructor when there is an opportunity to pass on a missing morsel, or should I say the right morsel - even to advanced students.
Just as the doctor takes our pulse, or our temperature, maybe even a blood test, and occasionally asks us to urinate in a cup - systemic things require systemic perspectives.
That's one of the energizing and intruiging things about teaching!