Inputs from the audio interface should be taken into account too. When using the External Insert facility, it is not possible to select a particular input if the same input is already selected as the input on any tracks within the project. Whenever I use the External Insert, I usually find that there are no available inputs as they were in use at an earlier stage when I was recording and they are set on tracks that are now hidden. I have to set about finding them, so I unhide the tracks and work through them in Track View to identify the ones with relevant inputs and setting them to "None". It is necessary to find them all, otherwise they don't show up in the External Insert's drop-down menu. It's all very tedious. There needs to be an easy way to see the routing and avoid having to sift through the tracks.

all these idea sort of hint the need for a new "view" in Sonar which shows all routing, maybe with blocks that can be collapsed and virtual cabling that can be changed by mouse ... and solo/mute states displayed and switchable
 
i would really like to see something like this because projects usually grow quite large, even before adding side chains and aux tracks/buses ...

Or hover mouse over the A, B, C label for a tooltip or toast type list. Or a text window.