In recent versions of Zynthian software (last update), you can see a tiny number in the bottom of each controller area. This tiny number is the MIDI CC assigned to the controller. Currently this works in all engines when the controller is assigned to a MIDI CC.
In MOD-UI, controllers are not assigned by default to any MIDI-CC, but you can do it by using the MIDI learning feature from the Zynthian UI, as described here:
MIDI learning: Currently, it’s working only in the MOD-UI engine, but will be implemented in all the engines very soon. For using the MIDI learning feature you need the touch interface. From the control screen, slide left-to-right over the desired controller frame and move the desired external controller. The captured MIDI-CC will be shown in the bottom of the control frame. For unlearning, slide right-to left.
I plan to improve the MIDI learning feature very soon, extending to all engines.
Enjoy!