It could be handled many ways.
A NEW dot for in a ref
Highlight if in model - and dot if used in the model and Ref.
Then a quick on and off for colors of models.
That is one thing that has not ben discussed.
That seeing each model in a transparent color of that model and only THE design in full glory.
(Or even better a transparent shade of the color with a tint of color for each ref. Just do it sequentually no need to define each just have a tinted color (still shows a tint of the actual color of the original) Make the active file stand out. And wil multiple screens you could have one shaded for each ref and one with everything.