If you have 24VAC power adapter connected, you can measure the AC (note: AC, not DC) voltage from the COM (common) terminal to a zone. When that zone is open, the voltage should be about 24VAC.
Alternatively, you can measure the DC voltage (note, DC) from +5V pin to a zone. When that zone is open, the voltage should be about 5VDC, and when the zone is closed, the voltage should be close to 0V.