The signals needed don’t exist on the OSPi. You would need to add capability to monitor current or output voltage. Output voltage would require sensing each output. If you only have one zone on at a time you could peak detect by using a diode to each output and monitor the single combined signal.
A better way (IMHO) is to monitor total current for the system. One sensor is needed. Then you can detect additional things such as open valve coils, bad wiring to valves, etc and the additional wiring is simplified as well. You would need to measure what normal current is for each combination of zones that you will use. Then apply some tolerance to get a window of acceptable values. This calibration step could be automated. The current sensor would need to be added and read with an A/D.
Here’s a link with various suggestions including this one:
http://www.avrfreaks.net/forum/detecting-open-wiringcoils-sprinkler-system