Do you have an oscilloscope? If so, you can connect it to any of the signal lines with ground (for example, CLK and GND) to monitor the signal.
If not, here is a hack that allows you to observe the signals: unplug the 8-pin cable from the OpenSprinkler Pi, and insert an LED (ideally with a current limiting resistor but I assume ok without) between a signal line with GND (for example, CLK and GND). Then you need to modify the code to insert delays (at least 1 second) between each toggling of the CLK line. This will allow you to watch the signals sent from RPi.