You can turn on debug prints. To do so, open defines.h, at the top, uncomment the line that says ‘#define ENABLE_DEBUG’. Save, and recompile the firmware.
To see the prints, you need to run the firmware manually. First, stop the background task by:
sudo /etc/init.d/OpenSprinkler.sh stop
then in the folder where you installed OpenSprinkler firmware, do:
sudo ./OpenSprinkler