Yes, a switch on the carriage and a switch on each dock wold be ideal. The carriage switch would confirm that a tool is loaded, and the dock switches would indicate which one it was.
I've been working with my tool changer yesterday and today. I've managed to get stall homing working on the coupler, and I am developing a set of configuration files for RRF3 on Duet 2. I'll also be moving the origin to the centre of the bed, which is what I use on all my 3D printers.