I also use the range dispay addon.
With it I can just mouse over any unit or unitframe to see approx how far away they are from me.
To know if alts are out of range of the mob I either see them not casting or glance at their windows and see the out of range spam at the top of their windows.
Or before pulling I can have my alts assist me and look at my alts bars to see if the spell icon is lit up for being in range or not.